Prof.    John   S.    Tatlock 


SELECT    WORKS 


OF 


JOHN     T  Y  N D  A  L L 

n 


FORMS  OF  WATER. 
LESSONS  IN  ELECTRICITY, 
SIX  LECTURES  ON  LIGHT. 


NEW    YORK; 

JOHN    B.    ALDEN,    PUBLISHER, 

1886, 


Tff 


FORMS  OF  WATER. 


BY 

JOHN  TYNDALL. 


AUCTION. 

\    v    Clou.!?,  Hair,s.  and  Rivers S  t 

:{.  Too  Waves  of  Li?.ht SO 

•i.  The  Wavej  cf  He  it  which  produce  the 
VapLi-  of  our  Atmosphere  and  ineit 

our  Glaciers 80 

5.  Experiments  to  prove  the    foregoing 

statements 88 

6    Oceanic  1  UstiUation   tf9 

7.  Tropical  Rains   90 

8.  Mountain  Condensers 91 

9.  Architecture  of  Snow 92 

10.  Atomic  Poles , 92 

1 1.  Architecture  of  Lake  Ice 94 

li'i.  The  Source  of  the  Arveiron.    Ice  Pin- 
nacles, Towers,   and  Chasms  of  the 
Glacier   des    Bois.     Passage    to   the 
Montanvert 91 

13.  The  Mer  de  Glace  and  its  Sources. 

Our  First  Climb  to  the  Cleft  Station.. .  93 

1 1.  Ice-cascade  and  Snows  of  the  Col  du 

Geant 90 

15.  Questioning  tue  Glaciers  97 

1(3.  Branches  and  Medial  Moraines  of  the 
Mer  de  Glace  from,  the  Cleft  Station. . .  £7 

1".  The  Talefre  and  the  Jardin.  Work 
among  the  Crevasses 98 

1.1  First  Questions  regarding  Glacier  Mo- 
tion. Drifting  of  Bodies  Buried  in 
Crevassa 93 

19.  Tho  Motion  of  Glaciers.  Measurements 
by  Hugi  and  AgaseLf.  Drifting  of 
Huts  on  the  Ico 100 

i.0.  Precise  Measurements  of  Agassiz  and 
Forbes.  Motion  of  a  Glacier  proved 
to  resemble  the  Motion  of  a  River 100 

•1\.  The  Theodolite  and  its  Use.  Our  own 

Measurements 101 

22.  Motion  of  the  Mer  de  Glace 101 

23.  Unequal  Motiou  of  the-  two  feides  of 
the  Mer  de  Glace .103 

24.  Suggestion  of  a  new  Likene-ss  of  Gla- 
cier Motiou  to  Biver  Motion.     Con- 
jecture tested   ...    104 

25.  New  Law  of  Glacier  Motion 106 

26.  Motion  of  Ax>a  of  Mer  de  Glace  106 

27.  Motion  of  Tributary  Q-laxjiens IQn 

2*.  Motion  ot  Top  r.nd  Bottom  of  Glacier.  .1^5 
29.  Lateral  Compression  of  a  Gi-iccer 106 


SECTJ.ON.  TAGS. 

u').  Longitudinal  Compression,  of  a  Glacier  IOC 
ol.  Sliding  and  Flowing.  Hard  Ice  and 

Soft  l^e 107 

3>.  Winter  ou  the  Mer  d 3  Glace 107 

33.  Winter  Motion  of  the  Mer  do  Glace . . . .  108 
84.  Motion  of  th3  Grindelwaid  and  Aletsch 

Glacier Kh 

35.  Motion  of  Morteratsch  Glacier 109 

36.  Birth  of  a  Crevasse:    .Reflections lv>9 

37.  Icicles 110 

38.  The  Bergscnrund     110 

;-9.  TransvfcToO  Crevasses Ill 

40.  Marginal  Crevasses Ill 

41.  Longitudinal  Crevasses 112 

42.  Crevasses  in  relation,  to  Curvature  of 
Glacier     112 

43.  Mor nine-ridge>,    Glacier   Tablc-s,    and 
Sand  Conej   . ,  113 

4  4    The  Glacier  Mills  or  Moulins 114 

4')   Tne  Changes  of  Volume  of  Water  l.y 

Heat  and  Cold 115 

43.  Consequences  flowing  from   the  fore- 
gning  f  roperties  of  Water.    Correction 

of  Errors " 116 

47-  The  Molecular  Mechanism   of  Water- 
Congelation  11  f> 

4S.  The  Dirt  Bands  of  the  Mer  de  Glace. .  .117 

•al)    Sea-ice  and  Icebergs 119 

50.  The   JEggischhorn,    the  MurgelLa    See 

and  its  icebergs ,  119 

01.  The  Bel  Alp 121 

r>2.  The  Kitfelherg  and  Gorner  Glacier.. ..  121 

;~>3.  Ancient  Glaciers  of  Switzerland 122 

54.  Erratic  Block* 123 

f>5.  Ancient  Glaciers  of  Engkmd,  Ireland, 

Scotland,  acd  Wales 123 

56.  The  Glacier  Epoch 1 24 

57.  Glacial  Theories 125 

58.  Dilation  and  Sliding   Theories 125 

59.  Plastic  Theory  12 "» 

00.  Viscous  Theory 1  -(> 

61.  Regelation  Theory 127 

02.  Cause  of  Ke^eiatkm   12* 

63.  Faraday's  View  of  Regelation ...  129 

64.  The  Uluc  Veins  of  Glaciers  ISO 

05  Relation  of  Mructur^  io  Pressure 132 

0<5.  Slate  Cleavage  and  Glacier  Lamination  133 
07.  Conclusion   ,,  ....13 


ivi300S91 


THE  FORMS  OF  WATER 


IN 


CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS, 

BY 

JOHN  TYNDALL,  LL.D.,  F.R.S., 

PROFESSOR   OF   NATURAL   PHILOSOPHY   IN   THE   ROYAL   INSTITUTION,    LONDON. 

WITH  NINETEEN  ILLUSTRATIONS  DRAWN  UNDER  THE  DIRECTION 

OF  THE  AUTHOR. 


PREFACE  TO  THE  FOURTH  EDITION. 

AT  a  meeting  of  the  Managers  of  the  Royal 
Institution  held  on  December  12th,  1825, 
"  the  Committee  appointed  to  consider  what 
lectures  should  be  delivered  in  the  Institu- 
tion in  the  next  session,"  reported  "  that  they 
had  consulted  Mr.  Furaday  on  the  subject  of 
engaging  him  to  take  a  part  in  the  juvenile 
lectures  proposed  to  be  given  during  the 
Christmas  and  Easter  recesses,  and  they 
found  his  avocations  were  such  that  it 
Would  be  exceedingly  inconvenient  for  him 
to  engage  in  such  lectures." 

At  a  general  monthly  meeting  of  the  mem- 
bers of  the  Royal  Institution,  held  on  Do- 
cember  4th,  1826,  the  Managers  reported 
"  that  they  had  engaged  Mr.  \Vallis  to  deliver 
a  course  of  lectures  on  Astronomy,  adapted 
to  a  juvenile  auditory,  during  the  Christmas 
vacation." 

In  a  report  dated  April  16th,  1827,  the 
Board  of  Visitors  express  "  their  satisfaction 
at  finding  that  the  plan  of  juvenile  courses 
of  lectures  has  been  resorted  to.  They  feel 
sure  that  the  influence  of  the  Institution  can- 
not be  extended  too  far,  and  the  system  of 
•nstructing  the  younger  portion  of  the  com- 
munity is  one  of  the  most  effective  means 
Which  the  Institution  possesses  foi  the  diffu- 
:2on  of  science." 

Faraday's  holding  aloof  was  but  tempo- 

IS",  for  at  Christmas.   1827,  we  iiud  him 


giving  a  "  Course  of  Six  Elementary  Lectures 
on  Chemistr}',  adapted  tc  a  Juvenile  Audi- 
tory."* 

The  Easter  lectures  were  soon  abandoned  ; 
but  from  the  date  here  referred  to  to  the  pres- 
ent time  the  Christmas  lectures  have  been  a 
marked  feature  of  the  Royal  Institution. 

In  1871  it  fell  to  my  lot'to  give  one  of  these 
courses.  I  had  been  frequently  invited  to 
write  on  Glaciers  in  encyclopedias,  journals, 
and  magazines,  but  had  always  declined  to 
do  so.  I  had  also  abstained  from  making 
them  the  subject  of  a  course  of  "lectures, 
wishing  to  take  no  advantage  of  my  position 
here,  and  indeed  to  avoid  writing  a  line  or 
uttering  a  sentence  on  the  subject  for  whicb 
1  could  not  be  held  personally  responsible. 
In  view  of  the  discussions  which  the  subject 
had  provoked,  I  thought  this  the  fairest 
course. 

But,  in  1871,  the  time  (I  imagined)  had 
come,  when,  without  risk  of  offence,  I  might 
tell  our  young  people  something  about  the 
labors  of  those  who  had  unravelled  for  their 
instruction  the  various  problems  of  the  ice- 
world.  My  lamented  friend  and  ever-helpful 
counsellor,  Dr.  Bence  Jones,  thought  the 
subject  a  good  one,  and  accordingly  it  was 
chosen.  Strong  in  my  sympathy  with  youth, 
and  remembering  the  damage  done  by  defec- 
tive exposition  to  my  own  young  mind,  I 
sought,  to  the  best  of  my  abiiity,  to  confer 
upon  these  lectures  clearness,  thoroughness, 


84 


THE  FORMS    OF  WATER 


and  life. 

Wishing,  moreover,  to  render  them  of  per- 
manent value,  I  wrote  out  copious  Notes  ol 
the  course,  and  had  them  distributed  among 
the  boys  and  girls.  In  preparing  these  Notes 
I  aimed  at  nothing  less  than  presenting  to  my 
youthful  audience,  in  a.  concentrated  but 
perfectly  digestible  form,  every  essential 
point  embraced  in  the  literature  of  the  gia- 
ciers,  and  some  things  in  addition,  which, 
derived  as  they  were  from  my  own  recent 
researches,  no  book  previously  published  on 
the  subject  contained. 

E'.H  my  theory  of  education  agrees  with 
th*»5  v>f  JSmeraoii.  according  to  which  instruc- 
tion is  only  half  the  battle,  what  he  calls 
provocation  being  the  other  half.  By  Ihis  he 
means  that  power  of  the  teacher,  through 
the  force  of  his  character  and  the  vitality  of 
his  thought,  to  bring  out  all  the  latent 
strength  of  his  pupil,  and  to  invest  with  in- 
terest even  the  driest  matters  of  detail.  In 
the  present  instance  I  was  determined  to 
shirk  nothing  essential,  however  dry  ;  and, 
to  keep  my  mind  alive  to  the  requirements 
of  my  pupil,  I  proposed  a  series  of  ideal 
rambles,  in  which  he  should  be  always  at  my 
side.  Oddly  enough,  though  1  was  here 
dealing  with  what  might  be  called  the  ab- 
stract idea  of  a  boy,  I  realized  his  presence 
so  fully  as  to  entertain  for  him,  before  our 
excursions  ended,  an  affection  consciously 
warm  and  real. 

The  Notes  here  referred  to  were  at  first 
intended  for  the  use  of  my  audience  alone. 
At  the  urgent  request  of  a  friend  I  slightly 
expanded  them,  and  converted  them  into  the 
little  book  here  presented  to  the  reader. 

The  amount  of  attention  bestowed  upon 
the  volume  induces  me  to  give  this  brief 
history  of  Us  origin. 

A  German  critic,  whom  I  have  no  reason 
to  regard  as  specially  favorable  to  me  or  it, 
makes  the  following  remark  on  the  style  of 
the  book  :  "  This  passion  [for  the  mountains] 
tempts  him  to  reveal  more  of  his  Alpine 
wanderings  than  is  necessary  for  his  demon- 
strations. Thu  reader,  however,  will  not 
find  this  a  disagreeable  interruption  of  the 
couise  of  thought;  for  the  book  thereby 
gains  wonderfully  in  vividness."  This,  I 
would  say,  was  the  express  aim  of  the  breaks 
referred  to.  I  desired  to  keep  my  companion 
fresh,  as  well  as  instructed,  and  these  inter- 
ruptions were  so  many  breathing-places 
where  the  intellectual  tension  was  purposely 
relaxed  and  the  mind  of  tlie  pupil  braced  to 
fresh  action. 

Of  other  criticisms,  flattering  and  other- 
wise, I  forbear  to  speak.  A>5  regards  some 
ftf  them,  indeed,  it  would  be  a  reproach  to 
that  manliness  which  I  have  sought  to  en- 
courage in  my  pupil  to  return  blow  for  blow. 
If  the  reader  be  acquainted  with  them,  this 
will  let  him  know  how  I  regard  them  ;  and 
if  he  be  not  acquainted  with  them,  I  wouM 
recommend  him  to  ignore  them,  and  to  form 
his  own  judgment  of  this  book.  No  fair- 
minded  person  who  reads  it  will  dream  that 
I,  in  writing  it,  had  thought  of  acting  other- 


wise than  justly  and  generously  toward  K:? 
predecessors,  the  last  of  whom,  1o  the  grief 
of  all  who  knew  him,  has  recently  passed 
away.  JOHN  TYNDALL. 

APBIL,  1874. 

§  1.  CLOUDS,  RAINS,  AND  RIVERS. 

1.  EVERY  occurrence  in  Nature  is  preceded 
by  otner  occurrences  which  arc  its  causes, 
and  succeeded  by  others  which  arc  its  effects. 
The  human  mind  is  not  satisfied  with  observ- 
ing and  studying  any  natural  occurrence, 
alone,  but  takes  pleasure  in  connecting  every 
natural  fact  with  what  has  gone  before  if. 
and  with  what  is  to  come  after  it. 
_  2.  Thus,  when  we  enter  upon  the  study  of 
rivers  and  glaciers,  our  interest  will  bo  great- 
ly augmented  by  taking  into  account  not 
only  their  actual  appearances,  but  also  their 
causes  and  effects. 

o.  Let  us  trace  a  river  to  its  source.  Be- 
ginning where  it  empties  itself  into  the  sea, 
and  following  it  backward,  we  find  it  from 
time  to  tinu  joined  by  tributaries  which 
swell  its  waters.  The  river  of  course  be- 
comes smaller  as  these  tributaries  are  passed. 
It  shrinks  first  to  a  brook,  then  to  a  stream  ; 
this  again  divides  itself  into  a  number  of 
smaller  streamlets,  ending  in  mere  threads  of 
water.  These  constitute  the  source  of  tlu 
river,  and  are  usually  found  among  hills.  j 

4.  Thus  the  Severn  lias  its  source  in  th.>' 
Welsh  Mountains  ;  the  Thames  in  the  Cots- 
wold  Hills  ;  the  Danube  in  tin?  hills  of  tlie 
Black  Forest  ;  the  Rhine  and  the  Rhone  in 
the    Alps  ;     the    Ganges    in    the    Himalaya 
Mountains  ;  the  Euphrates  near  Mount  Ara 
rat  ;  the  Garonne  in  t.'ie  Pyrenees  ;  the-  Elbe 
in  the  Giant  Mountains   of   I5oheni.':i  ;    tho 
Missouri  in  the  Rocky  Mountains,  and  the 
Amazon  in  the  Andes  of  Peru. 

5.  But  it  is  quite  plain  that  we  have  not 
yet  reached  the  real  beginning  of  the  liters. 
Whence  do  the  earliest  streams  derive  their 
water?    A  brief  residence  among  tlie  moun- 
tains would  prove  to  you  that  they  are  fed 
by  rains.     In  dry  weather  you  would  find 
the  streams  feeble,  sometime?  indeed  quitu 
dried  up.     In  wet  weather  yc'i  would   see 
them   foaming  torrents.      la   general   these 
streams  lose  themselves  as  littlo  threads  of 
water  upon  the  hill-sides  ;  but.  srmcliuies  you 
may  trace  a  river  to  a  definite  spring.     Tho 
river  Albula  in  Switzerland,    for    instance, 
rushes  at  its  origin  in  considerable  volumo 
from  a  mountain-side.     But  you  very  soon 
assure  yourself  that  such  springs  are  also  fed 
by  rain,  which  has  percolated  through  the 
rocks  or  soil,  and  which,  through  some  ori- 
fice that  it  has  found  or  formed,  comes  to  the 
light  of  day. 

0.  But  we  cannot  end  here.  Whence 
comes  the  rain  which  forms  the  mountain 
streams?  Observation  enables  you  to  an- 
swer the  question.  Rain  does  not'come  from 
a  clear  sky.  It  comes  from  clouds.  But 
wThat  are  clouds  ?  Is  there  nothing  you  are 
acquainted  with  which  they  resemble  ?  You 
discover  at  once  a  likeness  between  them  and 
the  condensed  steam  of  a  locomotive.  'At 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


85 


every  puff  of  the  engine  a  cloud  is  projected 
into  the  air.  Watch  the  cloud  sharply  :  you 
notice  that  it  first  forms  at  a  little  distance 
from  the  top  of  the  funnel.  Give  close  at- 
tention and  you  will  sometimes  see  a  per- 
fectly clear  space  between  the  funnel  and  the 
cloud.  Through  that  clear  space  the  thing 
which  makes  the  cloud  must  pass.  What, 
then,  is  this  thing  which  at  one  moment  is 
transparent  and  invisible,  and  at  the  next 
moment  visible  as  a  dense  opaque  cloud  ? 

7.  It  is  the  steam  or  xapor  of  water  from 
the  boiler.     Within  the  boiler  this  steam  is 
transparent  and  invisible  ;  but  to  keep  it  in 
this  invisible  state  a  heat  would  be  required 
as  great  as  that  within  the  boiler.     When  the 
vapor  mingles  with  the  cold  air  above  the  hot 
funnel  it  ceases  to  be  vapor.     Every  bit  of 
steam  shrinks,  when  chilled,  to  a  much  more 
minute  particle  of  water.     The  liquid  parti- 
cles thus  produced  form  a  kind  of  water-dust 
of  exceeding  fineness,  which  floats  in  the  air, 
and  is  called  a  cloud. 

8.  Watch  the  cloud-banner  from  the  fun- 
nel  of   a  running  locomotive  ;    you   see   it 
growing  gradually   less   dense.       It    finally 
nuilts  away  altogether,  and  it'  you  continue 
your  observations  you  will  not  fail  to  notice 
that  the  speed  of  its  disappearance  depends 
upon  the  character  of  the  day.     In  humid 
weather  the  cloud  hangs  long  and  lazily  in 
the  air  ;  in  dry  weather  it  is  rapidly  licked 
up.     What  has  become  of  it?    It  luis  been 
reconverted  into  true  invisible  vapor. 

9.  The  drier  the  air,  and  the  Iwtter  the  air, 
the  greater  is  the  amount  of  cloud  which 
can  be  thus  dissolved  in  it.     When  the  cloud 
first  forms,  its  quantity  is  far  greater  than  the 
air  is  able  to  maintain  in  an  ^invisible  stale. 
But  as  the  cloud  mixes  gradually  with  a 
larger  mass  of  air  it  is  more  an :1  more  dis- 
solved, and  finally  passes  altogether  from  the 
condition  of  a  finely-divided  liquid  into  that 
of  transparent  vapor  or  gas. 

10.  Make  the  lid  of  a  kettle  air-tight,  and 
permit  the  steam  t^>  issue  from  the  pipe  ;  a 
cloud  is  precipitated  in  all  respects  similar  to 
that  issuing  from  the  funnel  of  the  locomo- 
tive. 

11.  Permit  the  steam  as  it  issues  from  the 
pipe  to  pass  through  the  flame  of  a  spirit- 
lamp,  the  cloud  is  instantly  dissolved  by  the 
heat,  and  is  not  again  precipitated.     With  a 
special  boiler  and  a  special  nozzle  the  exper- 
iment may  be  made  more  striking,  but  not 
more  instructive,  than  with  the  kettle. 

12.  Look  to  your  bedroom  windows  when 
the  weather  is  very  cold  outside  ;  they  some- 
times stream  with  water  derived  from  the 
condensation  of  the  aqueous  vapor  from  your 
own  lungs.      The  windows  of"  railway  car- 
riages in  winter  show  this  condensation  in  a 
striking  manner.     Tour  cold  water  into  a  dry 
drinking-glass  on  a  summer's  day  :  the  out- 
fci'iu  surface  of  the  glass  becomes  instantly 
uunmccl  by  the    precipitation   of  moisture. 
On  a  warm  day  you  notice  no  vapor  in  front 
of  your  mouth,  but  on  a  cold  day  yi-u  form 
there  a  little  cloud  derived  from  ihe  conden- 
sation of  the  aqueous  vapor  from  the  hir^r. 


13.  You  may  notice  in  a  ball-room  that  as 
long  as  the  door    and    windows    are    kept, 
closed,  and  the  room  remains  hot,  the  air  re- 
mains clear  ;  but  when  the  doors  or  windows 
are  opened  a  dimness  is  visible,  caused  by 
the  precipitation  to  fog  of  the  aqueous  vapor 
of  the  ball-room.     If  the  weather  be  intensely 
cold  the  entrance  of  fresh  air  may  even  cause 
»now  to  fall.     This  has  been  observed  in  Rus- 
sian ball-rooms  ;  and  also  in  the  subterranean 
stables   at  Erzeroom,    when   the  doors   are 
opened  and  the  cold  morning  air  is  permitted 
to  enter. 

14.  Even  on  the  driest  day  this  vapor  is 
never    absent  from  our  atmosphere.      The 
vapor  diffused  through  the  air  of  this  room 
may  be  congealed  to  hoar  frost  in  your  pres- 
ence     This  is  done  by  filling  a  vessel  with,  a 
mixture  of  pounded  ice  and  salt,  which  is 
colder  than  the  ice  itself,  and  which,  there- 
fore,   condenses    and    freezes    the   aqueous 
vapor.     The  surface  of  the  vessel  is  finally 
coated  with  a  frozen  fur,  so  thick  that  it  may 
he  scraped  away  and  formed  into  a  snow- 
ball. 

15.  To  produce  the  cloud,  in  the  case  of 
the  locomotive  and  the  kettle,  heat  is  neces- 
sary.    By  heating  the  wakr  we  first  convert 
it  into  steam.  &n<l  then,  by  chilling  the  steam 
we  convert  it  into  cloud.     Is  there  any  file  in 
nature  which  produces  the  clouds  of  our  at- 
mosphere V    There;  is  :  the  fire  of  the  sun. 

10.  Thus,  by  tracing  backward,  without 
any  break  in  ihe  chain  of  occurrences,  our 
river  from  its  end  t»  its  real  beginnings,  we 
come  at  length  to  the  sun. 


IT.  There  are,  however,  rivers  which  have 
sources  somewhat  different  from  those  just 
mentioned.  They  do  not  begin  by  driblets 
on  a  hill-side,  nor  can  they  be  traced  to  a 
spring.  Go,  for  example,  to  the  mouth  of 
the  river  Rhone,  and  trace  it  backward  to 
Lyons,  where  it  turns  to  the  cast.  Bending 
round  by  Chambery,  you  come  at  length  to 
the  Lake  of  Geneva,  from  which  the  liver 
rushes,  and  which  }rou  might  be  disposed  to 
regard  as  the  source  of  the  Rhone.  But  go 
to  the  head  of  ihe  lake,  and  you  find  that 
the  Rhone  there  enteis  it,  that  the  lake  is 
in  fact  a  kind  of  expansion  of  the  liver. 
Follow  this  upward  ;  you  find  it  joined  by 
smaller  rivers  from  the  mountains  right  and 
left.  Pass  these,  and  push  your  journey 
higher  still.  You  come  at  length  to  a  lingo 
mass  of  ice — the  end  of  a  glacier — which  fills 
the  Rhone  valley,  and  from  the  boiuuii  of  tiie 
glacier  the  river  rushes  In  the  glacier  of 
the  Rhone  you  thus  find  the  souice  of  the 
river  Rhone. 

18.  But  again  we  have  not  reached  the  real 
beginning  of  the  river.  You  soon  convince 
yourself  that  this  earliest  water  of  the  Rhone 
is  produced  by  the  melting  of  the  ice.  You 
get  upon  the  glacier  and  walk  upward  along 
it.  After  a  time  the  ice  disappears  and  you 
come  upon  snow.  If  you  are  a  competent 
mountaineer  you  may  go  to  the  very  top  of 
this  great  snow-field,  and  if  you  cross  the  top 


THE  FORMS  OF  WATER 


and  descend  at  the  other  side  you  finally  ault 
the  snow,  and  get  upon  another  glacier  called 
the  Trift,  from  the  end  of  which  rushes  a 
river  smaller  than  the  Rhone. 

19.  You   soon    learn  that    the    mountain 
snow  feeds  the  glacier.     By  some  means  or 
other  the  snow  is  converted  into  ice.     But 
whence  comes  the  snow  ?    Like  the  rain,  it 
comes  from  the  clouds,  which,  as  before,  can 
be  traced  to  vapor  raised  by  the  sun.     With- 
out solar  lire  AVC  could  have  no  atmospheric 
vapor,    without    vapor  no  clouds,    without 
clouds  no  snow,  and  without  snow  no  gla- 
ciers. Curious  then  as  the  conclusion  maybe, 
the  cold  ice  of  the  Alps  has  its  origin  in  the 
heat  of  the  sun. 

§  3.  THE  WAVES  OF  LIGHT. 

20.  But  what  is  the  sun  ?    We  knov/  its 
size  and  its  weight.     We  also  know  that  it 
is  a  globe  of  fire  far  hotter  than  any  fire 
upon  earth.     But  we  now  enter  upon  another 
inquiry.    We  have  to  learn  definitely  what  is 
the  meaning  of  solar  light  and  solar'heat ;  in 
what  way  they  make  themselves  known  to 
our  senses  ;  by  what  means  they  get  from 
the  sun  to  the  earth,  and  how,  when  there, 
they  produce  the  clouds  of  our  atmosphere, 
and  thus  originate  our  rivers  and  our  glaciers. 

21.  If  in  a  dark  room  you  close  your  eyes 
and  press  the  eyelid  with  your  finger-nail,  a, 
circle  of  light  will  be  seen  opposite  to  the 
point  pressed,  while  a  sharp  blow  upon  the 
eye  produces  the  impression  of  a  flash  of 
light.     There  is  a  nerve  specially  devoted  to 
the  purposes  of  vision  which  comes  from  the 
brain  to  tho  back  of  the  eye,  and  there  di- 
vides into  line  filaments,  which  are  woven 
together  to  a  kind  of  screen  called  the  retina. 
The  retina  can  be  excited  in  various  ways  so  J 
ns  to  produce  the  consciousness  of  light ;  it 
m?,y,  as  we  have  seen,  be  excited  by  the  rude^ 
mechanical  action  of  a  blow  imparted  to  the 
eye. 

22.  There  is  no   spontaneous  creation  of 
light  by  the  healthy  eye.     To  excite  vision 
tho  relLia   must  be  affected  by  something  , 
coming  from  without.     What  is  that  some- 
thing?   In  some  way  or  other,    luminous 
bo:lies  have  the  power  of  affecting  the  retina 
— but  hoio  ? 

23.  It  was  long  supposed  that  from  such 
bodies  issued,  with  inconceivable  rapidity,  au 
inconceivably     fine     matter,     which    flew 
through  spaca,  passed  through  the  pores  sup- 
posed to   exist  in  the  humors  of  the  eye, 
reached  the  retina  behind,  and  by  their  shock 
.-gainst  the  retina,  aroused  the  sensation  of 
light. 

2i.  This  theory,  which  was  supported  by 
the  greatest  men,  among  others  by  Sir  Isaac 
Newton,  was  found  competent  to  explain  a 
great  number  of  the  phenomena  of  light,  but 
it  was  not  found  competent  to  explain  all  the 
phenomena.  As  the  skill  and  knowledge  of 
experimenters  increased,  large  classes  of  facts 
were  revealed  which  could  only  be  explained 
by  assuming  that  light  was  produced,  not  by 
a  fine  matter  Hying  through  space  and  hitting 
the  retina,  but  by  the  shock  of  minute  waxes 


against  the  retina.  .. 

25.  Dip  your  finger  into  a  basin  of  water, 
and  cause  it  to  quiver  rapidly  to  and  fro. 
From  the  point  of  disturbance  issue  small 
ripples  which    are  carried  forward  by  the 
water,  and   which  finally  strike  the  basin. 
Here,  in  the  vibrating  tinger,  you   have   a 
source  of  agitation  ;  in  the  water  you  have  a 
vehicle  through  which  the  finger's  motion  is 
transmitted,  and  you  have  finally  the  side  of 
the  basin  which  receives  the  shock  of  the 
little  waves. 

26.  In  like  manner,  according  to  the  warn 
theory  of  light,  you  have  a  source  of  agitation 
in  the  vibrating  atoms,  or  smallest  particles, 
of  the  luminous  body  ;  you  have  a  vehicle  of 
transmission  in  a  substance  which  is  sup- 
posed to  fill    all  space,  and  to   be  diffused 
through  the  humors  of  the  eye  ;  and  finally, 
you  have  the  retina,  which  receives  the  suc- 
cessive shocks  of  the  waves.     These  shocks 
are  supposed  to  produce  the  sensation  of  light. 

27.  We  are  here  dealing,  for  the  most  part, 
with  suppositions  and  assumptions  merely. 
We  have  never  seen  the  atoms  of  a  luminous 
body,  nor  their  motions.      We  have  never 
seen    the    medium    which    transmits    their 
•motions,  nor    the    waves  of  that  medium. 
How,  then,  do  \vc  come  to  assume  their  ex- 
istence ? 

;  28.  Before  such  an  idea  could  have  taken 
any  real  root  in  the  human  mind,  it  must  have 
been  well  disciplined  and  prepared  by  obser- 
vations and  calculations  of  ordinary  wave- 
motion.  It  was  necessary  to  know  how  both 
water- waves  and  sound-waves  arc  formed 
and  propagated.  It  was  above  all  thing.-.' 
necessary  to  know  how  waves,  passing 
through  the  same  medium,  act  upon  each 
other.  Thus  disciplined,  the  mind  was  pre- 
pared to  detect  any  resemblance  presenting 
itself  between  the  action  of  light  and  that  of 
waves.  Great  classes  of  optical  phenomena 
accordingly  appeared  which  could  be  ac- 
counted for  in  the  most  complete  and  satis- 
factory manner  by  assuming  them  to  be  pro- 
duced by  waves,  and  which  could  not  bo 
otherwise  accounted  for.  It  is  because  of  its 
competence  to  explain  all  tho  phenomena  of 
light  that  the  wave  theory  now  receives  uni- 
versal acceptance  on  tho  part  of  scientific 
men. 

Let  me  use  an  illustration.  We  infer  from 
the  flint  implements  recently  found  in  such 
profusion  all  over  England  and  in  other 
countries,  that  they  were  produced  by  men, 
and  also  that  the  Pyramids  of  Egypt  were 
built  by  men,  because,  as  far  as  our  expe- 
rience goes,  nothing  but  men  could  form  such 
implements  or  build  such  Pyramids.  In  like 
manner,  we  infer  from  the  phenomena  of 
light  the  agency  of  waves,  because,  as  far  as 
our  experience  goes,  no  other  agency  could 
produce  the  phenomena. 

§  4.  THE  WAVES  OF  HEAT  WHICH  PRODUCE 
THE  VAPOR  OF  OUR  ATMOSPHERE  AND 
MELT  OUR  GLACIERS. 
29.  Thus,  in  a  general  way,  I  have  given 

you  the  conception  and  the  grounds  of  tho 


IN  CLOUDS  ASTD  RIVERS,  ICE  AND  GLACIERS. 


87 


Fia.  1.— CLOUD-BANNER  OF  THK  AIGUILLE  DU  BRU  (par.  64  and  227X- 


conception,  which  regards  light  a~s  the  pro- 
duct of  wave-motion  ;  but  we  must  go  far- 
ther than  ihis,  and  follow  the  conception  into 
some  of  its  details.  We  have  all  seen  the 
waves  of  water,  and  we  know  they  are  of 
different  sizes — different  in  length  and  differ- 
ent in  height.  When,  therefore,  you  are  told 
that  the  atoms  of  the  sun,  and  of  almost  all 
other  luminous  bodies,  vibrate  at  diffcrci:: 
rates,  and  produce  waves  of  different  sizes 
your  experience  of  water-waves  will  enable 
you  to  form  a  tolerably  clear  notion  of  what 
is  meant. 

30.  As  observed  above,  we  have  never  seen 
the  light- waves,  but  we  judge  of  their  pres- 
ence, their  position,  and  their  magnitude,  by 
their  effects.  Their  lengths  have  been  thus 
determined,  and  found  to  vary  from  about 
•  to  ^ooouth  of  an  inch. 


31.  But  besides  those  which  produce  light 
the  sun  sends  forth  incessantly  a  multitude 
of  waves  which    produce   no"  light.      Ths 
largest  waves  which  the  sun  sen'is  forth  r.rc 
of  this  non-luminous  character,  though  they 
possess  the  highest  heating  power. 

32.  A  common  sunbeam  contains  waves  of 
all  kinds,  but  it  is  possible  to  ftift  or  filter  the 
beam  so  as  to  intercept  all  its  light,  and  to 
allow  its  obscure  heat  to   pass   unimpeded. 
For  substances  have  been  discovered  which, 
while  intensely  opaque  to  the  light- waves, 
are  almost  perfectly  transparent  to^he  others. 
On  the  other  hand,   it  is  possible,    by  the 
choice  of  proper  substances,  to  intercept  in  a 
great  degree   the   pure  heat-waves,    and   to 
allow  the  pure  light-waves  free  transmission. 
This  last  separation  is,  however;  not  so  per- 
fect as  the  h'rst. 


THE  FORMS  <jF  WATER 


33.  We  shall  learn  presently  how  to  detach 
the  one  class  of  waves  from  the  other  class, 
and  to  prove  that  v/aves  competent  to  light  a 
fire,  fuse  metal,  or  burn  the  hand  like  a  hot 
solid,  may  exist  in  a  perfectly  dark  place. 

34.  Supposing,  then,  that  we  withdraw,  in 
the  first  instance,  the  large  heat-waves,  and 
allow  the  light- waves  alone  to  pass.     These 
nviy  be  concentrated  by  suitable  lenses  and 
sent  into  water  without  sensibly  warming  it. 
Let  the  light -waves  now  be  withdrawn,  and 
tiie  larger  heat-waves  concentrated  in   the 
same  manner  ,  they  may  be  caused  to  boil 
the  water  almost  instantaneously. 

35.  This  is  the  point  to  which  I  wished  to 
ead  you.  and  which  without  due  preparation 
could  not  l>e  understood.      You  now   per- 
ceive the  important,  part   played   by   these 
large  darkness-waves,  if  I  may  use  the  term, 
in  the   work  of   evaporation.      When   they 
plunge  into  seas,  lakes,  and  rivers,  they  are 
intercepted  close  to   the  surface,  and  they 
heat  the  water  at  the  surface,  thus  causing 
it  to  evaporate  ;  the  light-waves  at  the  samd 
time  entering  to  great  depths  without  sensibly 
heating  the  water  through  which  they  pass. 
Not  only,  therefore,  is  it  the  sun's  fire  which 
produces  evaporation,  but  a  particular  con- 
stituent of  that  lire,  the  existence  of  which 
you  probably  wrere  not  aware  of. 

30.  Further  it  is  these  self-same  lightless 
waves  which,  falling  upon  the  g'.aei .TS  of 
the  Alps,  melt  the  ice  and  prot'-.ico  all  Hie 
rivers  flowing  from  the  glaciers  ;  f;>i*  I  shall 
prove  to  you  presently  that  the  light-waves, 
even  when  concentrated  to  the  uttermost,  are 
unable  to  melt  the  most  delicate  hoar-lro^t  ; 
much  less  would  they  be  able  to  produce  tlu 
copious  liquefaction  observed  upon  the  glo- 
ciers. 

37.  These  large  lightless  waves  of  the  sun, 
as  well  as  the  heat-waves  issuing  1  _>m  non- 
luminous  hot  bodies,  are  frequently  called 
obscure  or  invisible  heat. 

We  have  here  an  example  of  the  nv.nner 
in  which  phenomena,  apparently  remote,  are 
connected  together  in  this  wonderful  system 
of  things  that  we  call  Nature.  You  cannot 
study  a  snow-Hake  profoundly  wiihout  being- 
led  back  by  it  step  by  step  to  the  constitution 
of  the  sun.  It  is  thus  throughout  Nature. 
All  its  parts  are  interdependent,  and  tho 
study  of  any  one  part  completely  would  really 
involve  the  study  of  all 

g  5.  EXPERIMENTS  TO  PROVE  VIIE  FORE- 
GOING STATEMENTS. 

38  Heat  issuing  from  any  source  not  visi- 
bly red  cannot  be  concentrated  so  as  to  pro- 
duce the  intense  effects  just  referred  to.  To 
produce  these  it  is  necessary  to  employ  the 
obscure  heat  of  a  body  raised  to  the  highest 
possible  state  of  incandescence.  The  sun  is 
such  a  body,  and  its  dark  heat  is  therefore 
suitable  for  experiments  of  this  nature. 

39.  But  in  the  atmosphere  of  London,  and 
for  experiments  such  as  ours,  the  heat-waves 
emitted  by  coke  raised  to  intense  whiteness 
by  a  current  of  electricity  are  much  more 
manageable  than  the  sun's  waves.  The  elec 


trie  light  has  also  the  advautage  that  rts  dark 
radiation  embraces  a  larger  proportion  of  the 
total  radiation  than  the  dark  heat  of  tiie  sun. 
In  fact,  the  force  01  energy,  if  1  may  use  the 
term,  of  the  daik  Avaves  of  the  electric  light 
is  fully  seven  times  that  of  its  lightwaves. 
The  electric  light,  therefore,  shall  be  em- 
ployed in  our  experimental  demons!  rat  ions. 

40.  From  Ibis  source  a  powerful  beam  u 
sent  throuirh  the  room,  revealing  its  track  by 
the  motes  floating  in  the  air  of  the  room  ;  for 
were   the   mo'es   entirely   absent  the    beam 
would  be  unseen.     It  falls  unon  a  concave 
mirror  (a  glass  one  silvered  behind  will  an- 
swer) and  is  gathered  up  by  the  mirror  into 
a  cone  of  reflected  rays  ;  the  luminous  apex 
of  the  cone,  which  is  the/ocw«  of  the  mirror, 
being  about  fifteen  inches  distant  from  its 
reflecting  SMI  face.      Let  us  uiaik  the*  focus 
accurately  by  a  pointer. 

41.  And  now  let  us  place  in  the  path  of 
the  beam  a  substance  perfectly  opaque    tr 
light.     This  substance  is  iodine  dissolved  m 
a  liquid  called  bisulphide  of   carbon.     Tho 
light  at  the  focus  instantly  vanishes  when 
the  dark  solution  is  introduced.     But  the  so 
iution  is  intensely  transparent  to  the  dark 
waves,  and  a  focus  of  such  waves  remains  m 
the  air  of  the  room  after  the  light  has  been 
abolished.     You  may  feel  the  heat  of  these 
waves  with  your  hand  ;  you  may  let  them 
fall  upon   a  thermometer,   and   thus  prove 
their  presence  ;    or,    best  of  all,   you   may 
cause  them  to  produce  a  current  of  electric- 
ity, which  Reflects  a  large  magnetic  needle. 
The  magni'  .do  of  the  deflection  is  a  measure 
of  the  htMVi 

42.  Cm   n  \jeel  now  is,  by  the  use  of  a 
more  power,  til  lamp,  and  a  better  mirror  (one 
silvered  in  front  and  with  a  shorter  focal  dis- 
tance), to  intensify  the  action  here  rendered 
so  sensible.     As  before,  the  focus  is  rendered 
strikingly  visible  by  the  intense  illumination 
of  the  dust  particles.     We  will  first  filter  the 
beam  so  as  to  intercept  its  dark  waves,  and 
then  permit  the  purely  luminous  waves  to 
sxert  their  utmost  power  on  a  small  bundle 
of  gun-cotton  placed  at  the  focus. 

4;j.  No  effect  whatever  b  produced.  The 
gun-cotton  might  remain  there  for  a  week 
without  ignition  Let  us  now  permit  the 
uunitered  beam  t:>  act  upon  the  cotton.  It 
is  instantly  dissipated  in  an  explosive  flash. 
This  experiment  proves  that  the  light-waves 
are  incompetent  to  explode  the  cotton,  while 
th3  waves  of  the  full  beam  are  compel ent  to 
do  so  ;  hence  we  may  conclude  that  the  dark 
waves  are  the  real  airents  in- the  explosion1. 

44.  But    this    conclusion   would   be   only 
probable  ;    for  it  might   be  urged  that   the 
mixture  of  the  dark  waves  an.l  the  light  waves 
is  necessary  to  produce  the  result.  Let  us  then, 
by  means  of  our  opaque  solution,  isolate  our 
dark  waves  and  converge  them  on  the  cotton. 
It  explodes  as  before. 

45.  Hence  it  is  the  dark  waves,  and  they 
only,  that  are  concerned  in  the  ignition  of 
the  cotton. 

46.  At  the  same  dark  focus  sheets  01  plati- 
num are  raised  to  vivid  redness  ;   zinc  is 


IN  CLOUDS  AND  RIVERS,    ICE  AND  GLACIERS. 


80 


burned  ur>  ,  paper  instantly  blazes  ,  magne- 
sium wire  is  ignited  ;  charcoal  witiim  a  ra- 
ceivt- r  coutainiTig  oxygen  is  set  binning  :  a 
diamond  similarly  placed  is  caused  to  glow 
like  a  star,  being  afterward  gradually  dissi- 
pated. And  all  this  while  the  air  at  the  fo- 
"iis  remains  as  cool  as  in  any  other  part  of 
the  room. 

47.  To  obtain  the  light-waves  we  employ 
a  clear  solution  of  alum  in  water  ;  to  obtain 
the  dark  waves  we  employ  tho  solution  of 
iodine    above  referred   to.      But   as   before 
stated  (32),  the  aluui.  is  not  so  perfect  a  tiller 
as  the  iodine  ;  /:or  it  uausmits  a  portion  of 
the  obscure  her  i. 

48.  Though    the    light-waves  here  prove 
their  incompetence  to  ignite  gun-cotton,  they 
arc  able  to  burn  up  black  paper  ;  or,  indeed, 
to  explode  the  cotton  when  it  is  blackened. 
The  white  cotton  does  not  absorb  the  light, 
and  without  absorption  we  have  no  heating. 
The  blackened  cotton  absorbs,  is  heated,  and 
explodes. 

49.  Instead  of  a  solution  of  alum,  we  will 
employ  for  our  next  experiment  a  cell  of  pure 
water,  through  which  the  light  passes  with- 
out  sensible   absorption.      At    the  focus  is 
placed  a  test-tube  also  containing  water,  thtt 
full  force  of   the  light  being  concentrated 
upon  it.     The  water  is  not  sensibly  warmed 
by  the  conoentiated   waves.      We  now  re- 
move the  cell  of  water  ;  no  change  is  visible 
in  the  beam,  but  the  water  contained  in  the 
test-tube  now  boils. 

50.  The  light-waves  being  thus  proved  in- 
effectual, and  the  full  beam  effectual,  we  may 
infer  that  it  is  the  dark  waves  that  do  the  woi  k 
of  heating.      But  we  clinch  our  inference  by 
employing  our  opaque  iodine  filter.     Placing 
it  on  the  path  of  the  beam,  the  light  is  en- 
tirely stopped,  but  the  water  boils  exactly  as 
it  did  when  the  full  beam  fell  upon  it. 

51.  The   truth  of   the   statement  made  in 
paragraph  34  is  thus  demonstrated. 

52.  And  now  with  regard  to  the  im  llinrj  of 
ice.     On  the  surface  of  a  flask  contain:^  a 
freezing  mixture  we  obtain  n  thick  fur  of  hoar- 
frost (Par.  14).     Sending  the  beam  through 
a  water-cell,  its  luminous  waveo  c,n;  concen- 
( rated  upon  the  surface  of  too  E.a*"^    Not  a 
spicula  of  the  frost  is  disso'l^ti.     ''* e  now 
remove  the  water-c«jll,  and  in  ,'i  moment  a 
patch  of  the  frozen  fur  as  le.rge  as  half-a-crown 
is  melted.     Hence,  inasmuch  as  the  full  beam 
produces  this  effect,  and  the  luminous  part 
of  the  beam  does  not  produce  it,  we  fix  upon 
the  dark  portion  the  melting  of  the  frost. 

53.  As  before,  we  clinch  this  inference  by 
concentrating  the  dark  waves  alone  upon  the 
Mask.     The  frost  is  dissipated  exactly  as  it 
was  by  the  full.  beam. 

54.  These  effects  are  rendered  strikingly 
visible  by  darkening  with  ink  the  freezing 
mixture  within  the  flask.     When  the  hoar- 
frost is  removed,  the  blackness  of  the  surface 
from  which  it  had  been  melted  comes  out  in 
strong    cor-  ~  st   with    the   adjacent    su.wy 
'«vt!t,eness.      f^hen  the  flask  itself,  instead  of 
the  -freezing  mixture,  is  blackened,  the  purely 
luminous  waves  bein£  absorbed  by  the  glass. 


w&rm  it  ;  the  glass  reacts  upon  the  frost  nnd 
melts  it.  Hence  the  wisdom  of  darkening, 
instead  of  the  ikisk  itself,  the  mixture  within 
the  flask. 

t  55.  This  experiment  proves  to  demonstra- 
tion the  statement  in  paragraph  30  :  that  it 
is  the  dark  waves  of  the  sun  that  nit-It  thy 
mountain  snow  and  ice,  and  originate  all  the 
rivers  derived  from  glaciers. 

There  are  writers  who  seem  to  regard 
science  as  an  aggregate  of  facts,  nnd  hence 
doubt  its  efficacy  as  an  exercise  of  the  rea- 
soning powers.  But  all  that  I  have  here 
taught  you  is  the  result  of  reason,  taking  its 
stand,  however,  upon  the  sure  basis  of  ob- 
servation and  experiment.  And  this  is  the 
spirit  in  which  our  further  studies  are  to  be 
pursued. 

§  o.  OCEANIC  DISTILLATION. 

56.  IN.e  sun,  you  know,  is  never  exactly 
overhead  in  England.     But  at  the  equator, 
and  within  certain  limits  north  and  south  of 
it.  the  sun  &t  certain  periods  of  the  year  is 
directly  overhead  at  noon.     These  limits  are 
called  the  Tropics  of  Cancer  and  of  Capri- 
corn.     Upon   the    belt    comprised  between 
these  two  circles  the  sun's  rays  fall  with  their 
mightiest  power  ;  for  here  they  shoot  directly 
downward,  and  heat  both  earth  and  sea  more* 
than  when  they  strike  slantingly. 

57.  When  the  vertical  sunbeams  strike  the-- 
land  they  heat  it,  and  the  air  in  contact  with, 
the  hot  soil  becomes  heated  in  turn.      But. 
when  heated  the  air  expands,  and  when  it 
expands  it  becomes  lighter.     This  lighter  air- 
rises,  like  wood  plunged  into  waler.lh  rough 
the  heavier  air  overhead. 

58.  When  the  sunbeams  fall  upon  the  sea 
the  water  is  warmed,  though  not  so  much  as. 
the  land.     The  warmed  water  expands,  be- 
comes thereby  lighter,  and  therefore  continues, 
to  float  upon  the  top.     This  upper  layer  of 
water  warms  to  some  extent  the  air  in  contact 
with  it,  but  it  also  sends  up  a  quantity  of 
aqueous  vapor  which,  being  far  lighter  than 
air,  helps  the  latter  to  rise.     Thus  both  from 
the  land  and  from  the  sea  we  have  ascending- 
currents  established  by  the  action  of  the  sun! 

59.  When  they  reach  a  certain  elevation  in. 
the  atmosphere,    these  currents  divide  and 
flow,  part  toward  the  north  and  part  toward 
the  south  ;  -jvliiie  from  the  north  and  the  south 
a  flow  of  heavier  and  colder  air  sets  in  to. 
supply  the  place  of  the  ascending  warm  air. 

60.  Incessant  circulation  is  thus  established 
in  the  atmosphere.     The  equatorial  air  and 
vapor  flow   above   toward    the    north    and 
south  poles,  while  the  polar  air  flows  below 
toward  the  equator.      The  two  currents  of 
air  thus  established  are  called  the  upper  and 
the  lower  trade-winds. 

61.  But    before  the  air  returns  from  the 
poles  great  changes  have  occurred.     For  the 
air  as  it  quitted  the  equatorial  regions  was 
laden  with  aqueous  vapor,  which  could  not 
subsist  in  the  cold  polar  regions.     It  is  there 
precipitated,   falling  sometimes  as   rain,    or 
more  commonly  as  snow.     The  land  near  the 
pole  is  covered  with  this  snow,  which  gives 


00 


THE  FORMS  OF  WATER 


birth  to  vast  glaciers  in  a  manner  hereafter  to 
be  explained. 

02.  It  is  necessary  that  you  should  have  a 
perfectly  clear  view  of  this  process,  for  great 
mistakes  have  been  made  regarding  the  man- 
ner in  which  glaciers  are  related  to  the  heat 
of  the  sun. 

63.  It  was  supposed  that  if  the  sun's  heat 
were  diminished,  greater  glaciers  than  those 
now  existing  would  be  produced.     But  the 
lessening  of  the  sun's  heat  would  infallibly 
diminish  the  quantity  of  aqueous  vapor,  and 
thus  cut  otf  the  glaciers  at  their  source.     A 
brief  illustration^will  complete  your  knowl- 
edge here. 

64.  lu  the  process  of  ordinary  distillation, 
the  liquid  to  be  distilled  is  heated  and  con- 
verted into  vapor  in  one  vessel,  and  chilled 
and  reconverted  into  liquid  in  another.     What 
has  just  been  stated  renders  it  plain  that  the 
earth  an  1  its  atmosphere  constitute  a  vast 
distilling  apparatus  in  which  the  equatorial 
ocean  piays  the  part  of  the  boiler,  and  the 
chill  regions  of  the  poles  the  part  of  the  con- 
denser.    In  this  process  of  distillation  /teat 
plays  quite  as  necessary  a  part  as  cold,  and 
before  Bishop  Heber  could  speak  of  "  Green- 
land's icy  mountains,"  the  equatorial  ocean 
had  to  be  warmed  by  the  sun.     We  shall 
have  more  to  say  upon  this  question  after- 
ward. 

ILLUSTRATIVE  EXPERIMENTS. 

05.  I  have  said  that  when  heated,  air  ex- 
pands. If  you  wish  to  verify  this  for  your- 
self, proceed  thus.  Take  an  empty  flask, 
Btop  it  by  a  cork  ;  pass  through  the  cork  a 
narrow  glass  tube.  By  heating  the  tube  in 
a  spirit-lamp  you  can  bend  it  downward,  so 
that  when  the  flask  is  standing  upright  the 
open  end  of  the  narrow  tuba  may  dip  into 
water.  Now  cause  the  flame  of  your  spirit- 
lamp  to  play  against  the  flask,  The  flam  3 
heats  the  glass,  the  glass  heats  the  air  ;  the 
air  expands,  is  driven  through  the  narrow 
tube,  and  issues  in  a  storm  of  bubbles  from 
the  water. 

66.  Were  the  heated  air  unconfmed,  it  woull 
rise  in  the  heavier  cold  air.     Allow  a  sun- 
beam or  any  other  intense  light  to  fall  upon 
a  white  wall  or  screen  in  a  dark  room.     Bring 
u  heated  poker,  a  candle,  or  a  gas-flame  un- 
derneath the  beam.     An  ascending  current 
rises  from  the  heated  body  through  the  beam, 
and  the  action  of  the  air  upon  the  light  is 
such  as  to  render  the  wreathing  and  waving 
of  the  current  strikingly    visible  upon  the 
screen.      When  the  air  is  hot  enough,  anJ 
therefore  light  enough,  if   entrapped    in  a 
paper  bag  it  carries   the  bag  upward,  and 
you  have  the  fire  balloon. 

67.  Fold  two  sheets  of   paper  into  two 
cones,  and  suspend  them  with  their  closed 
points  upward  from  the  end  of  a  delicate 
balance.      See  that  the  cones  balance  each 
other.     Then  place  for  a  moment  the  flame 
of  a  spirit-lamp  beneath  the  open  base  of  one 
of  them  ;  the  hot  air  ascends  from  the  lamp 
and  instantly  tosses  upward  the  cone  above  it. 

68.  Into  an  inverted  glass  shade  introduce 


a  little  simks.  Let  the  air  come  to  r-?st.  an  1 
then  simply  place  your  hand  at  (he  open 
mouth  of  the  shade/  Mimic  hurricanes  nre 
produced  by  the  air  warmed  by  the  hand, 
which  are  strikingly  visible  when  the  smoka 
is  illuminated  by  a  strong  light. 

69.  The  heating  of  the  tropical  air  by  the 
sun  is  indirect.     The  solar  beams  have  scarce- 
ly any  power  to  heat  the  air  through  which 
they  pass  ;  but  they  heat  the  laud  and  ocean, 
and  these  communicate  their  heat  to  the  air 
in  contact  with  them.     The  air  and  vapor 
start    upward   charged   with  the  heat  thus 
communicated. 

§  7.  TROPICAL  RAINS. 

70.  But  long  before  the  air  and  vapor  from 
the  equator  reach  the   poles,   precipitation 
occurs.      Wherever    a  hurnid  warm    wind 
mixes  with  a  cold  dry  one,  rain  falls.     In- 
deed the  heaviest  rains  occur  at  those  places 
where  the  sun  is  vertically  overhead.     We 
must  inquire  a  little  more  closely  into  their 
origin. 

71.  Fill  a  bladder  about  two  thirds  full  of 
air  at  the  sea-level,  and  take  it  to  the  summit 
of  Mont  Blanc.     As  you  ascend,  the  bladder 
becomes  more  and  more  distended  ;   at  the 
top  of  the  mountain  it  is  fully  distended,  and 
has  evidently  to  bear  a  pressure  from  within. 
Returning  to  the  sea-level  you  find  that  the 
tightness  disappears,  the  bladder  finally  ap- 
pearing as  flaccid  as  at  first. 

72.  The  reason  is  plain.     At  the  sea-level 
the  air  within  the  bladder  has  to  bear  the 
pressure  of   the   whole    atmosphere,   being 
thereby  squeezed  into  a  comparatively  small 
volume.      In  ascending  the   mountain,  you 
leave  more  and  more  of  the  atmosphere  be- 
hind ;    the  pressure  becomes  less  and  less, 
and  by  its  expansive  force  the  air  within  the 
bladder  swells  as  the  outside  pressure  is  di- 
minished.    At  the  top  of  the  mountain  the 
expansion  is    quite  sufficient  to  render  the 
bladder  tight,  the  pressure  within  being  then 
actually  greater  than  the  pressure  without. 
By  means  of  an  air-pump  we  can  show  the 
expansion  of  u  balloon  partly  filled  with  air, 
when  the  external  pressure  has  been  in  part 
removed. 

78.  But  why  do  I  dwell  upon  this  ?  Sim- 
ply to  make  plain  to  you  that  the  unconfined 
air,  heated  at  the  earth's  surface,  and  as- 
cending by  its  lightness,  must  expand  more 
and  more  the  higher  it  rises  in  the  atmos- 
phere. 

74.  And  now  I  have  to  introduce  to  you  a 
new  fact,  toward  the  statement  of  which  1 
have  been  working  for  some  time.  It  is 
this  :  The  ascending  air  is  chilled  by  its  expan- 
sion. Indeed  this  chilling  is  one  source  of 
the  coldness  of  the  higher  atmospheric  re- 
gions. And  now  fix  your  eye  upon  those 
mixed  currents  of  air  and  aqueous  vapor 
which  lise  from  the  warm  tropical  ocean. 
They  start  with  plenty  of  heat  to  preserve 
the  vapor  as  vapor  ;  but  as  they  rise  they 
come  into  regions  already  chilled,  and  they  are 
still  further  chilled  by  their  own  expansion. 
.The  consequence  mio;ht  be  foreseen.  Th« 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


load  of  vapor  is  in  great  part  precipitated, 
dense  clouds  are  formed,  their  particles  coa- 
lesce to  rain-drops,  which  descend  daily  in 
gushes  so  profuse  that  the  word  "  torren- 
tial "  is  used  to  express  the  copiousness  of 
the  rainfall.  I  could  shoTv  you  this  chilling 
by  expansion,  and  also  the  consequent  pre- 
cipitation of  clouds. 

75.  Thus   long   before   the    air  from   the 
equator   reaches   the  poles,  its   vapor   is   iu 
great   part   removed   from   it,    having  rede 
sccnded  to  the  earth  as  rain.     Still  a  good 
quantity  of  the  vapor  is    carried  forward, 
which  yields  hail,  rain,  and  snow  in  noUhem 
and  southern  lands. 

ILLUSTRATIVE   EXPERIMENTS. 

76.  I  have  said  that  the  air  is  chilled  during 
its  expansion.     Prove  this,  if  you  like,  thus. 
With  a  condensing  syringe,  you  can  force  air 
into  an  iron  box  furnished  with  a  stopcock, 
to  which  the  syringe  is  screwed.     Do  so  till 
the   density  of    the    air   within   the   box   is 
doubled  or  trebled.     Immediately  after  this 
condensation,  both  the  box  and  the  air  within 
it  are  waj°m,  and  can  be  proved  to  be  so  by 
a  proper  thermometer.    Simply  turn  the  cock 
and  allow  the  compressed  air  to  stream  into 
the  atmosphere.     The  current,  if  allowed  to 
strike  a  thermometer,  visibly  chills  it  ;    and 
with    other   instruments  the    chill    may  to 
made  more  evident  still.      Even  the  hand 
feels  the  chill  of  the  expanding  air. 

77.  Throw  a  strong  light,  a  concentrated 
sunbeam  for  example,  across  the  issuing  cur- 
rent ;     if    the   compressed    nir   be   oidinary 
humid  air,  you  sec  the  precipitation  of  a  liltlc 
cloud  by  the  chill  accompanying  the  expan- 
sion.    This  cloud-formation  may,  however, 
be  better  illustrated  in  the  following  way  : 

78.  In   a   darkened    room    send  a  Mrong 
beam  of  light  through  a  glass  tube  three  feet 
long  and  three  inches  wide,  stopped  at  its 
ends  by  glass  plates.     Connect  the  tube  by 
means  of  a  stopcock  with  a  vessel  of  about 
one  fourth  its  capacity,  from  which  the  air 
has  been  removed  by  an  air-pump.     The  ex- 
hausted cylinder  of  the  pump  itself  will  an- 
s\ver  capitally.      Pill   the    glass  lube  with 
humid  air  ;  then  simply  turn  on  the  stopcock 
which  connects  it  with  the  exhausted  vessel. 
Having  more  room  the  air  expands,  cold  ac- 
companies the  expansion,  find,   as  a  conse- 
quence, a  dense  and  brilliant  cloud  imme- 
diately fills  the  tube.     If  the  expeiiment  be 
made  ^for  yourself  aione,  you  may  see  the 
cloud  in  ordinary  daylight  ;  indeed,  the  brisk 
exhaustion  of  any  receiver  filled  with  humid 
air  is  known  to  produce  this  condensation. 

79.  Other  vapors  than  that  of  water  may 
be  thus  precipitated,  some  of  them  yielding 
clouds  of  intense  brilliancy,  and  displaying 
iridescences,  such  as  are  sometimes,  but  not 
frequently,  seen  in  the  clouds  floating  over 
the  Alps. 

8'J.  In  science,  what  is  true  for  the  small  is 
true  for  the  large.  Thus  by  combining  the 
conditions  observed  on  a  large  scale  in 
nature  we  obtain  on  a  small  scale  the  Dhe- 
nomena  of  atmospheric  clouds. 


§  8.  MOUNTAIN  CONDENSERS. 

81.  To  complete  our  view  of  the  process  of 
atmospheric  precipitation  we  must  take  into 
account  the  action  of  mountains.     Imagine 
a  south-west   wind   blowing  across  the  At- 
lantic  towaid    Ireland.      In  its  passage  it 
charges  itself  with  aqueous  vapor.     In  the 
south  of  Ireland  it  encounters  the  mountains 
of  Kerry  :  the  highest  of  these  is  Magilli- 
cuddy's  Reeks,  mar  Killatne}'.      Now  the 
lowest  stratum  of  this  Atlantic  wind  is  that 
which    is   most   fully  charged    with  vapor. 
When  it  encounters  the  base  of  the  Kerry 
mountains  it  is  tilted  up  and  flows  bodily 
over  them.     Its  load  of  \apor  is  therefore 
cairied  to  a  height,  it  expands  on  reaching 
the  height,  it  is  chilled  in  consequence  of  the 
expansion,  and  comes  down  in  copious  show- 
ers of  rain.     From  this,  in  fact,  arises  the 
luxuriant  vegetation  of  Killainey  ;   to  this, 
indeed,   the   lakes  owe  their  water  supply. 
The  cold  crests  of  the  mountains  also  aid  in 
the  work  of  condensation. 

82.  Note   the  consequence.     There    is    a 
town  called  C.diirciveen,  to  the  south-west  of 
Mcigillicudely's  Reeks,  at  which  observations 
of  the  rainfall  have  been  made,  and  a  good 
distance  further  to  the  north-east,  right  in 
the  course  of  the  south-west  wind,  there  is 
another  town,  called  Pottariington,  at  which 
observations  of  rainfall  have  also  been  made. 
But  before  the  wind  reaches  the  latter  station 
it  has  passed  over  the  mountains  of  Kerry 
juid  left  a  great  portion  of  its  moisture  be- 
hind it.     What  is  the  result?    At  Cahirci- 
wen,  as  shown  by  Dr.   Lloyd,   the  raini'all 
amounts  to  51)  inches  in  a  year,    while  at 
Portarlington  it  is  only  21  inches. 

80.  Again,  you  may  sometimes  descend 
from  the  Alps,  when  the  fall  of  rain  and 
snow  is  heavy  and  incessant,  into  Italy,  anil 
find  the  sky*  over  the  plains  of  Lombardy 
blue  and  cloudless,  the  wind  at  the  sam* 
time  blowing  over  tJie  plain  toward  the  Alps. 
Below  the  wind  is  hot  enough  to  keep  its 
vapor  in  a  perfectly  transparent  state  ;  but  it 
meets  the  mountains,  is  tilted  up,  expanded; 
and  chilled.  The  cold  of  the  higher  summits 
also  helps  the  chill.  The  consequence  is  that 
the  vapor  is  precipitated  as  rain  or  snow, 
thus  producing  bad  weather  upon  the 
heights,  while  the  plains  below,  flooded  with 
the  same  air,  enjoy  the  aspect  of  the  un- 
clouded summer  sun.  Clouds  blowing  from 
the  Alps  are  also  sometimes  dissolved  over 
the  plains  of  Lombardy. 

84.  In  connection  with  the  formation  of 
clouds  by  mountains,  one  particularly  in- 
structive effect  may  be  here  noticed.  You 
frequently  see  a  streamer  of  cloud  many 
hundred  yards  in  length  drawn  out  from  an 
Alpine  peak.  Its  steadiness  appears  perfect, 
though  a  strong  wind  may  be  blowing  at  the 
same  time  over  th3  mountain-head.  Why  is 
the  cloud  not  blown  away?  It  is  blown 
away  ;  its  permanence  is  qnly  apparent.  At 
ene  end  it  is  incessantly  dissolved,  at  the 
other  end  it  is  incessantly  renewed  :  supply 
and  consumption  being  thus  equalized,  the 


93  THE  FORMS  OF  WATER 

cloud  appears  as  changeless  as  tfee  mountain  duced  in  calm  air,  the  icy  particles  build 
to  which  it  seems  to  cling.  When  the  red  themselves  into  beautiful  stellar  shapes,  inch 
sun  of  the  evening  shines  upon  these  cloud-  star  possessing  six  rays.  There  is  no  rievia- 
Rtreamers  they  resemble  vast  torches  with  tion  from  this  type,  though  in  other  respects 
their  flames  blown  through  the  air.  the  appearances  of  the  snow  stars  are  inli- 

§  9.    A  RCniTECTUItE   OF    SttOW. 


8;>.  We  now  resemble  persons  who  have  by,  who  gave  numerous  drawings  of  them. 
climbed  a  difficult  peak,  and  thereby  earned  I  have  observed  them  in  midwinter  tilling  the 
the  enjoyment  of  a  wide  prospect,  Having  air,  and  loading  the  slopes  of  the  Alps.  Bi?t 
made  ourselves  masters  bf  the  conditions  in  England  they  are  also  to  be  seen,  and  no 
necessary  to  the  production  i»f  mountain  words  of  mine  could  convey  so  vivid  an  im- 
snow,  we  are  able  to  take  a  comprehensive  pression  of  their  beauty  as  the  annexed  cTraw- 
and  intelligent  view  of  the  phenomena  of  ings  of  a  few  of  them,  executed  at  Green- 
glaciers.  wich  by  Mr.  Glaisher. 

80.  A  few  words  are  still  necessary  as  to  90.  It  is  worth  pausing  to  think  what  won- 
the  formation  of  snow.  The  molecules  and  derful  woik  is  going  on  in  the  atmosphere 
atoms  of  ail  substances,  when  allowed  free  during  the  formation  and  descent  of  every 
play,  build  themselves  into  definite  and,  for  snow-shower  :  what  building  power  is 
the  most  pait,  "beautiful  forms  called  ens-  brought  into  play  !  and  how  imperfect  seem 
tals.  Iivm.  copper,  gold,  silver,  lead,  sulphur,  the  productions  of  human  minds  and  hands 
when  melted  and  permitted  to  cool  gradually,  when  compared  with  those  formed  by  Die 
all  show  this  crystallizing  power.  The  metal  blind  forces  of  nature  ! 

bismuth  shows*  it  in  a  particularly  stiiking  01.  But  who  ventures  to  call  the  forces  of 
manner,  and  when  properly  fused  and  solidi-  nature  blind?  In  reality,  when  we  spi-ak 
iie.'l,  self-built  crystals  of  great  size  and  thus  we  are  describing  our  own  condition. 
beauty  are  formed  of  this  metal.  The  blindness  is  curs  ;  and  what  we  really 

87.  If  you  dissolve  saltpetre  in  water,  and  ought  to  say,  and  to  confess,  is  that  our 
£lh»w  the  solution  to  evaporate  slowly,  you  powers  are  absolutely  unable  to  comprehend 
:nay  obtain  large  crystals,  for  no  portion  of  cither  the  origin  or  the  end  of  the  operations 
the  salt  is  converted  into  vapor.  The  water  of  nature. 

of  our  atmosphere  is  fresh,  though  it  is  de-  $2-  But  while  we  thus  acknowledge  ou  • 
lived  from  the  salt  sea.  Sugar  dissolved  in  limits,  there  is  also  reason  for  wonder  at  the 
water,  and  pei  milled  to  evaporate,  yields  extent  to  which  science  has  mastered  the 
crystals  of  sugar  candy.  Alum  leadily  crys-  system  of  nature.  From  age  to  age,  and 
tallizes  in  the  same  way.  Flints  dissolved,  from  generation  to  generation,  fact  has  bed! 
as  they  sometimes  are  in  nature,  and  permit-  added  to  fact,  and  law  to  law,  the  irue  inctb- 
led  to  crystallize,  yield  the  prisms  and  pyra-  od  and  order  of  the  Universe  being  thereb/ 
mids  of  rock  crystal.  Chalk  dissolved  *and  more  and  more  revealed.  In  doing  this  i-ci- 
crystallized  yields  Iceland  spar.  The  dia-  <nc:e  has  encountered  and  overthrown  vaiir,us 
mond  is  crystallized  carbon.  All  our  pre  forms  of  superstition  and  deceit,  of  credulity 
clous  stones,  the  ruby,  sapphire,  beryl,  topaz  and  imposture.  But  the  world  continually 
emerald,  are  all  examples  of  this  crystallizing  produces  weak  persons  and  wicked  persons; 
power.  and  as  long  as  they  continue  to  exist  side  by 


88.  You  have  heard  of  the  force  of  gravi-  feicje>  as  th^7  do  in  this  our  day,  very  deh. 
tation;  and  you  know  that  it  consists  of  an  ing  beliefs  will  also  continue  to  infest  the 
attraction   of  every   particle  of    matter  for  world. 

every  other  particle.     You  know  that  plan- 

ets and  moons  are  held  in  their  orbits  by  this  §  10-  ATOMIC  POLES. 

attraction.  But  gravitation  is  a  very  simple  93.  "  What  did  I  mean  when,  a  few  nv;, 
affair  compared  to  the  force,  or  rather  forces,  ments  ago  (88),  1  spoke  of  attracting  and  re- 
of  crystallization.  For  litre  ihe  ultima!-  pcllent  poles?"  Let  ine  try  to  answer  tl,  5* 
particles  of  matter,  inr  onceivably  small  as  question.  You  know  that  astronomers  ami 
they  arc,  show  themselves  possessed  of  at-  geographers  speak  of  the  earth's  poles,  an  I 
tractive  and  rcpelleu  poles,  by  the  mulual  you  have  also  heard  of  magnetic  poles,  tlx.> 
Action  of  which  the  shape  and  structuie  ol  poles  of  a  magnet  being  the  points  at  whi<  h 
the  crystal  are  determined.  In  the  soiid  con-  the  attraction  and  repulsion  of  the  magm  I 
dition  the  attracting  poles  are  rigidly  locked  are  as  it  v/ere  concentrated. 
together  ;  but  if  sufficient  heat  be  apolied  the  9-J-  Every  magnet  possesses  two  such 
b-uid  of  union  is  dissolved,  and  in  the  slate  poles  ;  and  if  iron  tilings  be  scattered  over  * 
of  fusion  the  poles  arc  pushed  so  far  asunder  magm-t,  ear-h  particle  becomes  al»o  endowed 
as  to  be  practically  out  of  each  other's  range,  with  two  poles.  Suppose  such  particles  de- 
The  natural  tendency  of  the  molecules  to  v«>d  of  weight  and  floating  in  our  atmos- 
bui'ld  themselves  together  is  thus  neutralized,  phere,  what  must  occur  when  they  come 

89.  This  is  the  case  with  water,  which  as  a  near  <-»ac!i  other?     Manifestly  the  repellent 
liquid  is  to  all  appearance  formless.     When  poles  will  retreat  from  each  other,  while  thy 
sutliciently  cooled  the  molecules  are  brought  attractive  poles  will  approach  and  finally  lock 
within  the  play  of  the  crystallizing  force,   themselves  together.       AIM!   supposing  the 
and  they  tbcp  -••'•nnrre  themselves  in  forms  of  particles,  instead  of  a  single  pair,  to  possess 
ir/'-escribabte  i,**.  When  snow  is  pio-   several  pairs  of  poles  arranged  at  definitj 


IK  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


FIG.  2.— SNOW  CRYSTALS. 


points  over  their  surfaces  ;  you  can  then  pic- 
ture them,  in  obedience  to  their  mutual  at- 
tractions and  repulsions,  building  themselves 
together  to  form  masses  of  definite  shape  and 
structure. 

(J5.  Imagine  the  molecules  of  water  in 
cairn  cold  air  to  be  gifted  with  poles  of  this 
description,  which  compel  the  particles  to  la}' 
themselves  together  in  a  detiaite  order,  anil 
you  have  before  your  mind's  eye  the  unseen 
architecture  which  finally  produces  the  visi-- 


ble  and  beautiful  crystals  of  tbf  r.aow. 
Thus  our  first  notions  and  conceptions  of 
poles  are  obtained  from  the  sight  ol'  o\ir  eyes 
in  looking  at  the  effects  of  magnetism  ;  and 
we  then  transfer  these  notions  and  concep- 
tions to  particles  which  no  eye  has  ever 
seen.  The  power  by  which  we  thus  picture 
to  ourselves  effects  beyond  the  range  of  the 
senses  is  what  philosophers  call  the  Imagina- 
tion, and  in  the  effort  of  the  mind  to  seize 
upon  the  unseen  architecture  of  crystals,  we 


T,iE  FORMS  O 


ATER 


have  an  example  of  the  "  scientific  use'  of 
this  faculty.  Without  Imagination  vye 
mi<rht  have  critical  power,  but  not  creative 
oower,  in  science. 

§11.    AllCLIITECTUHE   OF   L.\Ki:   ICE. 

OC.  We  have  thus  made  ourselves  acquaint- 
ed with  the  beautiful  snow-flowera  self-con- 
structeil  by  the  molecules  of  water  in  calm 
cold  air.  Do  the  molecules  show  this  archi- 
tectural power  when  ordinal y  water  is 
frozen  ?  What,  for  example,  h  the  structure 
of  the  ice  over  which  we  skate  in  winter? 
Quite  as  wonderful  as  the  lloweis  of  the 
snow.  The  observation  is  rare,  if  not  new, 
but  I  have  seen  in  water  slowly  freezing  six- 
rayed  ice-stars  formed,  and  floating  free  on 
the  surface.  A  six-rayed  star,  mm  cover,  b 
typical  of  the  construction  of  all  our  lake  ice. 
It  is  built  up  of  such  forms  wonderfully  in- 
terlaced. 

97.  Take  a  slab  of  lake  ice  and  place  ^it  in 
the  path  of  a  concentrated  sunbeam.     Watch 
the  track  of  the  beam  through  the  ice.     Part 
of  the  beam  is    stopped,    pait  of    it  goes 
through ;   the    former     produces     internal 
liquefaction,  the  latter  has  no  cifi-ct  what- 
ever upon  the  ice.     But  the  liquefaction  M 
not  uniformly  diffused.    From  separate  spots 
of  the  ice  little  shining  points  are   seen   to 
sparkle  forth.     Every  one  of  those  points  is 
surrounded  by  a  beautiful  liquid  llower  with 
six  petals. 

98.  Ice  and  water  are  so  optically  alike  that 
unless  the  light  fall  properly    upon    these 
tiowers  you  cannot  see  them.     But  what  is? 
the  central  spot  ?    A  vacuum.     Ice  swims  on 
water  because,  bulk  for  bulk,  it  is  lighter 
than  water  ;  so  that  when  ice  is  melted  it 
shrinks  in  size.     Can  the  liquid  flowers  then 
occupy  the  whole  space  of  the  ice  melted  ! 
Plainly  no.     A  little  empty  space  is  foim-j  I 
with  the  flowers,  and  this  space,  or  rather  its 
surface,  shines  in  the  sun  with  the  lustre  of 
burnished  silver. 

99.  In  all   cases  the  flowers  are  formed 
parallel  to  the  surface  of  freezing.     They  are 
formed  when  the  sun  shines  upmi  the  ice  of 
every  lake ;  sometimes  in  myriads,  and  so 
small  as  to  require  a  magnify  ing-glass  to  see 
them.     They  are  always  attainable,  but  then 
beauty  is  ofien  marred  by  internal  defects  of 
the  ice.     Even  one  portion  of  the  same  piece 
of  ice  may  show  them  exquisitely,  while  a 
second  portion  shows  them  imperfectly. 

100.  Annexed  is  a  very  imperfect  sketch 
of  these  beautiful  figures. 

101.  Here  we  have  a  reversal  of  the  pro 
cess  of  crystallization.     The  searching  solar 
beam  is  delicate  enough  to  take  the  molecules 
down  without  deranging  the  order  of  their 
architecture.     Try  the  experiment  for  vour- 
eelf  with  a  pocket-lens  on   a   sunny   day. 
You  will  not  find  the  flowers  confused  ;  they 
till  lie  parallel  to  the  surface  of  freezing.     In 
this  exquisite  wray  every  bit  of  the  u-e  over 
which  our  skaters  glide  in  AV  inter  is  put  to- 
gether. 

102.  I  said,  in  97,  that  a  portion  of  the 
sunbeam  was  .stopped  by, the  JQO  and  lique- 


fied it.  AVhat  ia  this  portion?  The  dark 
heat  of  the  sun.  The  great  body  of  the  light 
wavts  and  even  a  portion  of  the  d.irk  one* 
pass  thiough  the  ice  without  losing  any  of 
their  heating  power.  When  propeily  con- 
centrated on  combustible  bo-Jies,  even  after 
having  passed  thiough  the  ice,  their  burning 
power  becomes  manifest. 

103.  And  the  ice  itself  may  be  employed 
to  coneenttatu  th'-m.     With  an  ice-lens  i:> 
the  polar  regions  Dr.  Bcoresby  has  often  con- 
centrated the  sun's  rays  so  as  to  make  then 
burn  won«1,  fire  gunpowder,  and  melt  lead  ; 
thus  proving  that  the  limiting  power  is  re- 
tained   by  the  rays,    even  after    they  have 
passed  through  so  cold  a  substance. 

104.  By  rendering  the  rays  of  the  electric 
lamp    parallel,     and     then     sending     them 
through  a    lens   of   ice,  we    obtain  all   the 
effects  which  Dr.  Scoresby  obtained  with  the 
rays  of  the  sun. 

§  12.  Tnz  Gounca  OP  THE  ARVEIKON. — ICE 
PINNACLES,  TOWEIIS.  AND  CHASMS  OF  THIS 
GLACIS:!  Di:3  Co:3.— PASSAGE  TO  THE 

MONTANVCRT. 

105.  Our  preparatory  studies  arc  for  tho 
present  ended,  and  thus  informed,  let  us  ap- 
proach  the  Alps.     Through  the  village   of 
Chamouui,  in  'oavoy,  a  ri^er  rushes  which  U 
called  the  Arve.       Let    us  trace  this  rivjL-r 
backward  from  Chamouni.     At  a  little  dis- 
tance from  the  village  the  river  folks;  one 
of  its  branches  still  continues  to  Le  called  the 
Arve,  the  other  is  the  Arveiron.     Following 
this  latter  we  corne  to  what  is  called  the 
"  source  of  the  Arveiron" — a  short  hour's 
walk  from  Chamouni.     Here,  as  in  the  case 
of  the  Rhone  already  referred  to,  you  ara 
fronted  by  a  huge  mass  of  ice,  the  end  of  a 
glacier,  and  from  au    arch    in  the  ice  tho 
Arveiron  issues.     Do  not  trust  the  arch  ia 
summer.     Its  roof  falls  at  intervals  with  u 
startling  crash,  and  would  infallibly  crush 
any  person  on  whom  it  might  fall. 

106.  We  must  now  be  observant.     Look- 
ing about  us  here,  we  find  in  front  of  the  ice 
curious  heaps  and  ridges  of  debris,  which 
are  more  or  less  concentric.     These  are  the 
terminal  moraine*  of  the  glacier.     We  shall 
examine  them  subsequently. 

107.  We  now  turn  to  the  left,  and  ascend 
the  slope  beside  the  glacier.     As  we  ascend 
we  get  a  better  view,  and  find  that  the  ice 
here  fills  a  narrow  valley.     We  come' upon 
another  singular  ridge,  not  of  fresh  debris 
like  those  lower  down,  but  covered  in  part 
with  trees,  and  appearing  to  be  literally  as 
"old  as  the  hills."      It    tells  a  wonderful 
tale.       We  soon  satisfy  ourselves  that  the 
ridge  is  an  ancient  moraine,  and  at  once  con- 
clude that  the  glacier,  at  some  former  period 
of  its  existence,  was  vastly  larger  than  it  is 
now.       This    old    moraine    stretches    right 
across  the  main  vf.lley,  and  abuts  against  the 
mountains  at  the  opposite  side. 

108.  Having  passed  the  terminal  portion  of 
the  glacier,  which  is  covered  with  stones  and 
rubbish,   we  find    ourselves    beside    a  very 
wonderful  exhibition  of  ice.    The  glacier  de 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS.  95 

gcends  a  steep  gorge,  and  in  doing  so  is  riven  beautiful  pyramid  of  the  Aiguille  du  Dru 
and  broken  in  the  most  extract  dinary  man-  (shown  in  our  frontispiece)  ;  and  to  the  right 
ner.  Here  are  towers,  and  pinnacles,  and  at  the  Aiguille  des  Charrnox,  with  its  sharp 
fantastic  shapes  wrought  out  by  the  action  pinnacles  bent  as  if  they  were  ductile.  Look- 
of  the  weather,  which  put  one  in  mind  of  ing  straight  up  the  glacier  the  view  is  bound 
rude  sculpture.  From  deep  chasms  in  the  cd  by  the  great  crests  called  La  Granao 
glacier  issues  a  delicate  sh:mmer  of  blue  Jorasse,  nearly  14,000  feet  high.  Our  object 
lin-ht.  At  times  we  hear  a  sound  like  Hum-  now  is  to  get  into  the  very  heart  of  the 
der,  which  arises  either  from  the  falling  of  a  mountains,  and  to  pursua  to  its  origin  tne 
tower  of  ice  or  from  the  tumble  of  a  huge  wonderful  frozen  river  which  we  have  just 
stone  into  a  chasm.  Thw  *•  lacier  maintains  crossed. 

this  wild  and  chactic  cbaiaeler  for  some  114.  Starting  from  the  Montanvert  with 
time:  and  the  best  iceman  would  find  him-  the  glacier  below  in  to  our  left,  we  soon 
self  defeated  in  any  attempt  to  get  along  it.  reach  some  rocks  resembling  the  Mauvais 

109.  We  reach  a  place  railed  i"ne  Cbapeau,   Pas  ;  they  are  called  tea  Ponts.     We  cross 
where,  if  we  wish,,  we  can  have  Refreshment  them  and  reacli  I 'Annie,  where  we  quit  the 
in  a  little  mountain  hut.     We  then  pass  the  l:md  for  the  ice.     We  walk  up  the  glacier, 
MauwiiaPo*,  a  precipitous  rock,  on  the  face  but  before  i caching  the  promontory  called 
cf  which  steeps  are  hewn,  and  the  utiprac-  Trfilaporte,  we  take  once  more  to  the  moun- 
tiscd  traveller  is  assisted  by  a  lope.    \V<-  par-  tain-side  ;  for  though  the  path  here  has  been 
sue  our  journey,  partly  along  the  mountain-  forsaken  on  account  of  its  danger,  for  the 
side,  and  partly  along  a  ridge  of  singularly  sake  of  knowledge  w;  are  prepared  to  incur 
art.ficial  aspect— a  lateral   moraine.     We  at  danger  to  a  reasonable  extent.     A  little  gla- 
lungth  face  a  house  perched  upon  an  emi-  cier  reposes  on  the  slope  to  our  right.     Wq 
nonce  at  the  opposite  side   of  the  glacier.   may  see  a  huge  boulder  or  two  poised  on  the 
This  is  the  auberge  of  the  Moritanveit,  well  CQd  of  the  glacier,  and,  if  fortunate,  also  see 
Known  to  all  visitors  to  this  portion  of  the  the  boulder  liberated  and  plunging  violently 
Alps.  down  the  slope.     Presence  of  mind  is  all  that 

110.  Here  we  cross  the  glacier.     I  should  is  necessary  to  render  our  safety  certain  ;  but 
have  told  you  that  its  lower  part,  including  travellers  do  not  always  show  presence  of 
the  broken  portion  we  have  passed,  is  called  mind,  and  hence  the  path  which  formerly 
the  Glacier  des  Bois  ;  while  the  place  that  led  over  this  slope  has  been  forsaken.     The 
wo  are  now  about  to  cross  is  the  beginning  whole  slope  is  cumbered  by  masses  of  rock 
of   the   Met   de   Glace.     You  feel  that  this  which    this    little    glacier    has  sent  down, 
term  is  not  quite  appropriate,  for  the  glacier  These  I  wished  you  to  see  ;  by  and  by  they 
here  is  much  more  like  a  riser  of  ice 'than  a  sna11  be  fully  accounted  for. 

sea.  The  valley  which  it  fills  it  about  half  a  H5.  Above  Trelaportc  to  the  right  you  see 
mile  wide.  a  most  singular  cleft  in  the  rocks,  in  the 

111.  The  ice  maybe  riven  where  we  en-  middle  of  which  stands  an  isolated  pillar, 
ter  upon  it,  but  with  the  necessarv  care  there  hewn  out  by  the  weather.     Our  next  object 
is  no  difficulty  in  crossing  this  portion  of  the  is  to  get  to  the  tower  of  rock  to  the  left  of 
Mer  de  Glace.     The  clefts  and  chasms  in  the  tn^t  cleft,  for  from  that  position   we  shall 
ire  aie  called  crevasses;  we  shall  make  their  gain  a  must  commanding  and   instructive 
acquaintance  on  a  grander  scale  by  and  by.     view  of  the  Mer  de  Glace  and  its  sources. 

li'2.  Look  up  and  down  this  bide  of  the  116..  The  cleft  referred  to,  with  its  pillar* 
glacier.  It  is  considerably  riven,  but  as  we  maybe  seen  to  the  right  of  the  above  engrav- 
advance  the  crevasses  will  diminish,  and  we  ing  of  the  Mer  de  Glace.  Below  the  cleft 
*h»\\  find  ven*  few  of  them  at  the  other  side,  is  also  seen  the  little  glacier  just  referred 
Note  this  for' future  use.  The  ice  is  at  first  to. 

diily  ;  but  the  dirt  soon  disappears,  and  you  117.  We  may  reach  this  cleft  by  a  steep 
come  upon  the  clean  crisp  suiface  of  the  gla-  gully,  visible  from  our  present  position,  and 
cier.  You  have  already  noticed  that  Ihe  leading  directly  up  to  the  cleft.  But  these 
clean  ice  is  while,  and  thai  fiom  a  distance  gullies,  or  couloirs,  are  very  dangerous,  be- 
it  resembles  snow  rather  tlu.n  ice.  This  is  ing  the  path ways,of  stones  falling  from  the 
caused  by  the  breaking  up  of  the  surface  by  heights.  We  will  therefore  take  the  rocks  to 
the  solar  heat.  When  you  pound  trans-  the  left  of  the  gully,  by  close  inspection  as- 
parent  rock-salt  into  powder  it.  is  as  white  certain  their  assailabl'3  points,  and  there  at- 
as  table-salt,  and  it  is  the  minute  fissuring  tack  them.  In  the  Alps  as  elsewhere  won- 
</f  the  surface  of  the  glacier  by  the  sun's  rays  derful  things  may  bo  dona  by  looking  stead- 
that  causes  it  to  appear  white.  Within  the  fastly  at  difficulties,  and  testing  them  wher- 
glacier  the  ice  is  transparent.  After  an  ex-  ever  they  appear  assailable.  We  thus  reach 
hilarating  passage  we  get  upon  the  opposite  our  station,  where  the  glory  of  tlie  prospect, 
lateral  moraine,  and  ascend  the  steep  slope  and  the  insight  that  we  gain  as  to  the  forma- 
from  it  to  the  Montanvert  Inn.  tion  of  the  Mer  de  Gl  ice.  far  more  than  re- 

§    13.     THE    MER    DE    GLACE    AND    ITS   pay  us  for  thy  labor  of  our  ascent. 

SOURCES.— OUR    FIRST    CLIMB    TO    THE       I18-  For  we  see,  the    glacier  below  us, 

CLEFT  STATION  stretching  its  frozen  tongue  elownward  past 

the  Montanvert.     And  we  now  rind  this  sin- 

113.  Here  the  view  before  us  is  very  £le  glacier  branching  out  into  three  others, 
grand.  We  look  across  the  glacier  at  the  some  of  them  wider  than  itself.  Regard  the 


THE  FORMS  OF  WATEE 


V-  iin.'h  to  the  light,  the  Glacier  du  Ge:-mt. 
It. stretches  smoothly  up  for  a  long  distance, 
I  hen  becomes  disturbed,  and  then  changes  to 
a  great  frozen  cascade,  down  which  the  ice 
appears  to  tumble  in  wild  confusion.  Above 
the  cascade  you  see  an  expanse  of  shinmg 
snow,  occupying  an  area  of  some  square 
miles. 

§  14.  ICE-CASCADE    AND    SNOWS    OP    THE 

COL  DU  GEANT. 

119.  Instead  of  climbing  to  tho  height  where 
we  now  stand,  we  might  have  continued  our 
walk  upon  the  Mer  do  Gluvc  turned  round 


the  promontory  of  Teilap^itc,  and  walked 
right  up  the  Glacier  du  Geant.  We  should 
have  found  ice  under  our  feet  up  to  the  bottom 
of  the  cascade.  It  is  not  so  compact  as  the 
ice  lower  down,  but  you  would  not  think  of 
icf using  to  call  it  ice. 

120.  A  we  approach  the  fall,  the  smooth 
and  unbroken  character  of  the  glacier 
changes  more  and  more.  We  encounter 
transverse  ridges  succeeding  each  other  with 
augmenting  steepness.  The  ice  becomes 
more  and  more  fissured  and  confused.  We 
wind  through  tortuous  ravines,  climb  huge 
ice-mounds,  and  creep  cautiously  along 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


97 


crumbling  crests,  with  crevasses  right  and 
left.  The  confusion  increases  until  further 
advance  along  the  centre  of  the  glacier  is  im- 
possible. 

121.  But  with  the  aid  of  an  axe  to  cut 
steps  in  the  steeper  ice- walls  and  slopes  we 
might,   by  swerving  to  cither   side  of  the 
glacier,  work  our  way  to  the  top  of  tne  cas- 
cade.    If  we  ascended  to  the  right,  we  should 
have  to  take  care  of  the  ice  avalanches  which 
sometimes  thunder  down  the  slopes  ;  if  to  the 
left,  we  should  have  to  take  care  of  the  stones 
let  loose  from  the  Aiguille  Noire.     After  we 
had  cleared  the  cascade,  we  should  have  to 
beware  for  a  time  of  the  crevasses,  which  for 
some  distance  above  the  fall  yawn  terribly. 
But  by  caution  we  could  get  round  them,  and 
sometimes  cross  them  by  bridges  of  snow. 
Here  the  skill  and  knowledge  to  be  acquired 
only  by  long  practice  come  into  play  ;  and 
here  also  the  use  of  the  Alpine  rope  suggests 
itself.     For  not  only  are  the  snow-bridges 
often  frail,  but  whole  crevasses  are  some- 
times covered,  the  unhappy  traveller  being 
first  made  aware  of  their  existence  by  the 
snow  breaking  under  his  feet.    Many  lives 
have  thus  been    lost,   and  some  quite  re- 
cently. 

122.  Once  upon  the  plateau  above  the  ice- 
fall  we  find  the  surface  totally  changed.     Be- 
low the  fall  we  walked  upon  ice  ;  here  we  are 
upon  snow.     After  a  gentle  but  long  ascent 
we  reach  a  depression  of   (he  ridge  which 
hounds  the  snow-fi°ld  at  the  top,  and  now 
iook  over  Italy.     We  stand  upon  the  famous 
Col  du  Geant. 

123.  They  were  no  idle  soimpcrcrs  on  the 
mountains  that  made  these  wild  recesses  first 
Known  ;    it   was  not  the  desire  for  health 
which  now  brings  some,   or  the  desire  for 
grandeur  and  beauty  which  brings  others,  or 
the  wish  to  be  able  to  say  that  they  have 
climbed  a  mountain  or  crossed  a  col,  which 
I  ft'ar  brings  a  good  many  more  ;  it  was  a 
desire  for  knowledge  that  brought  the  first  ex- 
plorers here,  and  on  this  col  the  celebrated 
De  Saussurc  lived  for  seventeen  days,  mak- 
ing scientific  observations. 

§  15.  QUESTIONING  THE  GLACIERS. 

124.  I  would  now  ask  you  to  consider  for 
a  moment  the  facts  which  such  an  excur- 
sion places  in  our  possession.     The  snow 
through  which  we  have  in  idea  trudged  is 
iihe  snow  of  last  winter  and  spring.     Had  we 
placed  last  August  a  proper  mark  upon  the 
surface  of  the  snow,  we  should  find  it  this 
August  at  a  certain  depth  beneath  the  sur- 
fMce.     A  good  deal  has  been  melted  by  the 
summer  sun,  but  a  good  dual  of  it  remains, 
and  it  will  continue  until  the  snows  of  the 
coming  winter  fall  and  cover  it.     This  again 
will  be  in  part  preserved  till  next  August,  a 
good  deal  of  it  remaining  until  it  is  covered 
by  the  snow  of  the  subsequent  winter.     We 
thus  arrive  at  the  certain  conclusion  that  on 
the  plateau  of  the  Col  du  Geant  the  quantity 
of  snow  that  falls  annually  e.vcmt*  the,  quan- 
tity melted. 

125.  Had  we  conic  in  the  month  of  April 


or  May,  we  should  have  found  the  glacier  be- 
low the  ice  fall  also  covered  with  snow, 
which  is  now  entirely  cleared  away  by  the 
heat  of  summer.  Nay,  more,  the  ice  there 
is  obviously  melting,  forming  running  brooks 
wThich  cut  channels  in  the  ice,  and  expand 
here  and  there  into  small  blue-green  lakes. 
Hence  you  conclude  with  certainty  that  be- 
low the  ice-fall  the  quantify  of  frozen  material 
failing  upon  the  (/lacier  is  leas  than  the  quan- 
tity melted. 

126.  And  this  forces  upon  us  another  con- 
clusion ;  between  the  glacier  below  the  ice- 
fall  and  the  plateau  above  it  there  must  exist 
a  line  where  the  quantity  of  snow  which  falls 
is   exactly  equal    to  the    quantity    annually 
melted.      This  is  the  snow-line.      On  some 
glaciers  it  is  quite  distinct,  and  it  woidd  be 
distinct  here  were  the  ice  less  broken  and 
confused  than  it  actually  is. 

127.  The  French  term  neve  is  applied  to 
the  glacial  region  above  the  snow-line,  while 
the  word  glacier  is  restricted  to  the  ice  below 
it.     Thus  the  SUDWS  of  the  Col  du  G£ant 
constitute  the  nev6  of  the  Glacier  du  Geant, 
and  in  part,  the  neve  of  the  Mer  de  Glace. 

128.  But  if  every  year  thus  leaves  a  resi- 
due of  snow  upon  the  plateau  of  the  Col  du 
Geant,  it  necessarily  follows  that  the  plateau 
must  get  annually  higher,  provided  the  snow 
remain  upon  it.     Equally  certain  is  the  con- 
clusion that  the  whole  length  of  the  glacier 
below  the  cascade  must  sink  gradually  lowei , 
if  the  waste  of  annual  melting  be  not  made- 
good.     Supposing  two  feet  of  snow  a  year  to 
remain  upon  the  Col,  this  would  raise  it  to  & 
height  far  surpassing  that  of  Mont  Blanc  in 
five    thousand    years.     Such    accumulation, 
must  take  place  if  the  snow  remain  upon  the 
Col ;   but  the  accumulation  does  not  take 
place,  hence  the  snow  does  not  remain  on 
the  Col.    The  question  then  is,  Whither  does 
it  go? 

§  16.  BRANCHES   AND    MEDIAL   MORAINES 

OF  THE  MER  DE  GLACE  FROM  THE  CLEFT 

STATION. 

1.29.  We  shall  grapple  with  this  question 
immediately.  Meanwhile  look  at  that  ice- 
valley  in  fiont  of  us,  stretching  up  between 
Mont  Tacul  and  the  Aiguille  de  Lechaud, 
to  the  base  of  the  great  ridge  called  the 
Grande  Jorasse.  This  is  called  the  Glacier 
de  Lechaud.  It  receives  t*t  its  head  the 
snows  of  the  Jorasse  and  of  Mont  Mallet, 
and  joins  the  Glacier  du  Geunt  at  the  prom- 
ontory of  the  Tacul.  The  glacieis  seem 
welded  together  where  they  join,  but  they 
continue  distinct.  Between  them  you  clearly 
trace  a  stripe  of  debris  (c  on  the  annexed 
sketch-plan) ;  you  trace  a  similar  though 
smaller  stripe  (a  on  the  sketch)  from  the 
junction  of  the  Glacier  du  Gaunt  with  the 
Glacur  des  Periades  at  the  foot  of  the  Ai- 
guille Noire,  which  you  also  follow  along 
the  Mer  de  Glace. 

130.  We  also  see  another  glacier, or  a  portion 
of  it,  to  the  left,  falling  apparently  in  broken 
fragments  through  a  narrow  gorge  (Cascade 
dif  Talefre  on  the  sketch)  and  joining  the 


98 


THE  FORMS  OF  WATER 


Fia.  5.— SKETCH-PLAN,  SHOWING  THE  MORAINES  a,  b,  c,  d,  o,  or  THE  MER  DE  GI.ACE. 


Lechaud.  and  from  their  point  of  junction 
also  a,  stripe  of  d6bris  (d)  runs  downward 
along  the  Mer  de  Glace.  Beyond  this  again 
we  notice  another  stripe  (e),  which  seems  to 
begin  at  the  bottom  of  the  ice-fall,  rising  as 
it  were  from  the  bodv  of  the  glacier.  Beyond 
all  of  these  \ve  can  notice  the*  lateral  moraine 
of  the  Mer  de  Glace. 

131.  These  stripes  are  the  medial  moraines 
of  the  Mer  de  Glace.  We  shall  learn  more 
about  them  immediately. 

l':J2.  And  now,  having  informed  our 
minds  by  these  observations,  let  our  eyes 
'wander  over  the  whole  glorious  scene,  the 
spHn  lured  peaks  and  the  hacked  end  jagged 
crests,  the  far-stretching  snow-fields,  Ihe 
smaller  glaciers  which  nestle  on  the  heights, 
the  deep  blue  heaven  and  the  sailing  clouds. 
T-*  it  not  worth  some  labor  to  gain  command 
of  suHi  a  scene  ?  But  the  delight  it  imparts 
is  heightened  by  the  fact  that  we  did  not 
come  expressly  to  see  it ;  we  came  to  instruct 
ourselves  about  the  glacier,  and  this  high  en- 
1  >yment  is  an  incident  of  our  labor.  You 
will  find  it  thus  through  life  ;  without  hon- 
est labor  there  can  be  no  deep  joy. 
$  17.  THE  TALEFRE  AND  THE  JARDIN. — 

WOUK  AMONG  THE   CBEVASSES. 

1P>.°>.  And  now  let  us  descend  to  the  Mer 
de  Glace,  for  I  want  to  take  you  across  the 
Clacitr  to  that  broken  i-e-fall,  the  origin  of 


which  we  have  not  yet  seen.  We  aim  at  the 
farther  side  of  the  glacier,  and  to  reach  it  we 
must  cross  those  dark  stripes  of  debris  which 
we  observed  from  the  heights.  Looked  at 
from  above,  these  moraines  seemed  flat,  but 
now  we  find  them  to  be  ridges  of  stones  and 
rubbish,  from  twenty  to  thirty  feet  high. 

134.  We  quit  the  ice  at  a  place  called  the 
Couvercle,  and  wind  round  this  promontory, 
ascending  all  the  time.  We  squeeze  ourselves 
through  the  Ecjralets,  a  kind  of  natural  stair- 
case in  the  rock,  and  soon  afterward  obtain 
a  full  view  of  the  ice-fall,  the  origin  of  which 
we  wish  to  find.     The  ice  upon  the  fall  is 
much  broken  ;  we  have  pinnacles  and  towers, 
some  erect,  some  leaning,  and  some,  if  we 
are  fortunate,   falling  like  those  upon  the 
Glacier  des  Bois  ;  ond  we  have  chasms  f torn 
which  issues  a  delicate  blue  light.     With  ihe 
ice-fall  to  our  right  we  continue  to  ascend, 
until  at  length  we  command  a  view  of  a 
huge  glacier  basin,  almost  level,  and  on  the 
middle  of  which  stands  a  solitary  island,  en- 
tirely surrounded  by  ice.     We  stand  at  the 
edge  of  the  0 lacier  du  Talefre,  and  connect 
it  with  the  ice-fall  we    have  passed.     The 
glacier  is  bounded  by  rocky  ridges,  hacked 
and  torn  at  the  top  into  teeth  and  edges,  and 
buttressed  by  snow  fluted  by  the  descending 
stones. 

135.  We  cross  the  basin  to  the   central 
island,   and  find  grass  and  floweis  at  tii« 


IN  CLOUDS  AJSTD  RIVERS,  ICE  AND  GLACIERS. 


place  where  we  enter  upon  it.  This  is  the 
celebrated  Jardin,  of  which  you  have  often 
bear!.  The  upper  part  of  the  Jardin  is  bare 
rock.  Close  at  hand  is  one  of  the  noblest 
prvik.s  in  this  portion  of  the  Alps,  the  Ai- 
guille Vorie.  It  is  between  thirteen  and 
fourteen  thousand  feet  high,  and  down  its 
sides,  after  freshly- fallen  snow,  avalanches 
incessantly  thunder.  From  one  of  its  pro- 
jf-ctions  a  *tieak  of  moraine  starts  down  the 
Talc  f  re  ;  from  the  Jardin  also  a  similar  streak 
of  moraine  issues.  Both  continue  side  by  side 
to  the  top  of  the  ice-fall,  where  they  are  en- 
gulfed  in  the  chasms.  But  at  the  bottom  of 
the  fall  they  reappear,  as  if  newly  emerging 
from  the  body  of  the  glacier,  and  afterward 
they  continue  along  tin-;  Mer  de  Glace. 

13G.  Walk  with  me  now  alongside  the  mo- 
raine from  the  Jardin  down  toward  the  ice- 
fall.  For  a  time  our  work  is  easy,  such  fis- 
sures as  appear  offeiing  no  impediment  to 
our  march.  But  the  crevasses  become  grad- 
ually wider  and  wilder,  following  each  other 
at  length  so  rapidly  as  to  leave  merely  walls 
of  ice  between  them.  Here  perfect  steadi- 
ness of  foot  is  necessary — a  slip  would  be 
death.  We  look  toward  the  fall,  and  ob- 
serve the  confusion  of  walls  and  blocks  and 
chasms  below  us  increasing.  At  length  pru- 
dence and  reason,  cry  "Halt!"  We  may 
swerve  to  ihe  light  or  to  the  left,  and  mak- 
ing our  way  along  crests  of  ice,  with  chasms 
on  both  hands,  reach  either  the  right  lateral 
moraine  or  the  left  lateial  moraine  of  the 
glacier. 

§  18.  FIRST  QUESTIONS  REGARDING  GLA- 
CIER MOTION. — DUIFTISG  OF  BODIES 
BURIED  ix  A  CREVASSE. 

137.  But  what  arc  these  lateral  moraines? 
As  you  and  I  go  from  day  to  day  along  the 
efaeiers,  their  origin  is  gradually  made  plain. 
We  see  at  intervals  the  stones  and  rubbish 
descending  from  the  mountain-sides  and  ar- 
rested by  the  ice.     All  along  the  fringe  of 
the  glacier  the  stones  and  rubbish  fall,  and  it 
soon  becomes  evident  that  we  have  here  the 
source  of  the  lateral  moraines. 

138.  But  how  are  the  medial  moraines  to 
be  accounted    for?    How  does  the   debris 
range  itself  upon  the  glacier  in  stripes  some 
hundreds  of  yards  from  its  edge,  leaving  the 
space  between  them  and  the  edge  clear  of 
rubbish  ?    Some  have  supposed  the  stones  to 
have  rolled  over  the  glacier  from  the  sides, 
but  the  supposition  will  not  bear  examina- 
tion.    Call  to  mind  now  our  reasoning  re- 
garding the  excess  of  snow  which  falls  above 
the  snow-line,  and  our  subsequent  question, 
How  is  the  snow  disposed  of  ?    Can  it  be  that 
the  entire  mass    is   moving  slowly   down- 
ward ?    If  so,  the  lateral  moraines  would  be 
carried  along  by  the  ice  on  which  they  rest, 
and  when  two  branch   glaciers  unite  they 
would  lay  their  adjacent  lateral  moraines  to- 
gether to*  form  a  medial  moraine  upon  tha 
trunk  glacier. 

139.  There  is,  in  fact,  no  way  that  we  c:in 
*ee  of  disposi.ig  of  the  excess  of  snow  above 
the  snow-line  ;  there  is  no  way  of  making 


good  the  constant  waste  of  the  ice  below  the 
snow-line  ;  there  is  no  way  of  accounting  for 
the  medial  moraines  of  the  glacier,  but  by 
supposing  that  from  the  highest  snow-fields 
of  the  Col  -In  G6ant,  the  Lechaud,  and  the 
Talefre,  to  the  extreme  end  of  the  Glacier  des 
Bois,  the  whob  mass  of  frozen  matter  i.s 
moving  downward. 

140.  If  you  were  older,  it  wouM  give  me 
Treasure  to  take  you  up  Mont  Blanc.     Stait- 
Jig   from  Chamouai,   we  shoukl  first  pass 
through    woods  au  I  pastures,   then  up  tha 
steep  hill-face  with  tlie  Glacier  des  Bossons 
to  our  right,  t.->  a  rock  known  as  the  Pierre 
Pointae  ;  thence  to  a  higher  rock  called  th« 
Pierre  Vfichelle,    because   here    a-  la-Uer  is 
usually  placed  to  assist  iu  crossing  the  chasms 
of  the  glacier.     At  the  Pierre  ]'Ech°lle  we- 
should  strike  tin  ice,  and  passing  under  the 
Aiguille  du  Mi  li,  which  towers  to  the  Icjft, 
and  wh'ch  sonrrJmes  sweeps  a  portion  of 
the  track  with  stone  avalanches,  we  should 
cross  the  Glacier  des  Bossons  ;  amid  heaped- 
up  mounds  and  broken  towers  of  ice  ;  up 
steep  slopes  ;  over  chasms  so  deep  that  their 
bottoms  are  hid  in  darkness. 

141.  \Ve  reach  the  rocks  of  the  Grands 
Mulets,  which  form  a  kind  of  barren  islet  in 
the  icy  sea  ;  thence  to  the  higher-snow -fields, 
crossing  the  Petit  Plateau,  which  we  should 
find  cumbered  by  blocks  of  ice.     Looking  to 
the  right,  we  should  see  whence  they  came, 
for  rising  here  with  threatening  aspect  high 
above  us  arc  the  broken  ice-crags      of  the 
Dome  du  Goute.  The  guides  wish  to  pass  this 
place  in  silence,  and  it  is  just  as  well  to  hu- 
mor them,  however  much  you  may  doubt 
the  competence  of  the  human  voice  to  bring 
the  ice-crags  down.     From  the  Petit  Plateau 
a  steep  snow-slope  would  carry  us  to  the 
Grand  Plateau,  and  at  day-dawn  I  know 
nothing  in  the  whole  Alps  more  grand  and 
solemn  than  this  place. 

143.  One  object  of  our  ascent  would  be 
now  attained  ;  for  here  at  the  heail  of  the 
Grand  Plateau,  and  at  the  foot  of  the  final 
slope  of  Mont  Blanc,  I  should  show  you  a 
great  crevasse,  into  which  three  guides  were 
poured  by  an  avalanche  in  the  ytar  1820. 

143.  Is  this  language  correct  ?    A  crevasse 
hardly  to  be  distinguished  from  the  present 
one  undoubtedly  existed  here  in  1820.     But 
was  it  the  identical  crevasse  now  existing? 
Is  the  ice  riven  here  to-day  the  same  as  that 
riven  fifty-one   years  ago?    By  no  means. 
How  is  this  proved  ?    By  the  fact  that  more 
than  forty  years  after  their  interment,  the 
remains  of  those  three  guides  were  found 
near  the  end  of  the  Glacier  des  Bossons, 
many  miles  below  the  existing  crevasse. 

144.  The  same  observation  proves  to  dem 
onstration  that  it  is  the  ice  near  the  bottom  of 
the  higher  n6ve  that  becomes  the  surface-ice 

of  the  glacier  near  its  end.  The  waste  of  the 
surface  below  the  snow-line  brings  the  deeper 
portions  of  the  ice  more  and  more  to  the 
light  of  day. 

145.  There  are  numerous  obvious  indica- 
tions of  the    existence  of  glacier    motion, 
though  it  is  too  sl&w  to  catch  the  eyo  at, 


100 


THE  FORMS  OF  WATER 


once.  The  crevasses  change  within  certain 
]imits  from  year  to  year,  and  sometimes  from 
month  to  month  ;  and  this  could  not  be  if 
the  ice  did  not  move.  Rocks  and  stones  also 
are  observed,  which  have  been  plainly  torn 
from  the  mountain-sides.  Blocks  seen  to 
fall  from  particular  points  are  afterward  ob- 
served lower  down.  On  the  moraines  rocks 
are  found  of  u  totally  different  mineralogical 
character  from  those  composing  the  moun- 
tains right  and  left  ;  and  in  all  such  cases 
strata  of  the  same  character  are  found  bor- 
dering the  glacier  higher  up.  Hence  the 
conclusion  that  the  foreign  boulders  have 
been  floated  d  ,wn  by  the  ice.  Further,  the 
ends  or  "  snouts"  of  many  glaciers  act  like 
ploughshares  on  the  land  in  front  of  them, 
overturning  with  slow  but  merciless  energy 
huts  and  chalets  that  stand  in  their  way. 
Facts  like  these  have  been  long  known  to  the 
inhabitants  of  the  High  Alps,  who  were  thus 
made  acquainted  in  a  vague  and  general  way 
with  the  motion  of  the  glaciers. 

§  ID.  THE  MOTION  OF  GLACIERS. — MEASURE- 
MENTS BY  HUGI  AND  AQASSIZ. — DRIFTING 
OF  HUTS  ON  THE  ICE. 
140.  But  the  growth  of  knowledge  is  from 
vagueness  toward  precision,  and  "exact  de- 
terminations of  the  rate  of  glacier  motion 
were  soon  desired.  With  reference  to  such 
measurements,  one  glacier  in  the  Bernese 
Oberlaud  will  remain  forever  memorable. 
From  the  little  town  of  Meyringen  in  Swit- 
zerland you  proceed  up  the  valley  of  Hasli, 
past  the  celebrated  waterfall  of  Handeck, 
where  the  river  Aar  plunges  into  a  chasm 
more  than  200  feet  deep.  You  approach  the 
Grimsel  Pass,  but  instead  of  crossing  it  you 
turn  to  the  right  and  follow  the  course  of  "the 
Aar  upward.  Like  the  Rhone  and  the 
Arveiron,  you  find  the  Aar  issuing  from  a 
glacier. 

147.  Get  upon  the  ice,  or  rather  upon  the 
deep  moraine  shingle  which  covers  the  ice, 
and  walk  upward.     It  is  hard  walking,  but 
af'.er  some  time  you  get  clear  of  the  rubbish, 
and  on  to  a  wide  glacier  with  a  great  medial 
moraine  running  along  its  back.     This  mo- 
raine is  formed  by  the  junction  of  two  branch 
glaciers,   the  Lauteraar   and  the  Finsteraar, 
which  unite  at  a  promontory  called  the  Ab- 
Rchwung  to  form  the  trunk  glacier  of  the 
Unteraar. 

148.  On  this  great  medial  moraine  in  1827 
an  intrepid  and  enthusiastic  Swiss  professor, 
Ilugi,  or  Solothurm  (French  Soleure),  built 
a  hut  with  a  view  to  observations  upon  the 
glacier.     His  hut  moved,  and  he  measured 
its  motion.     In  the  three  years— from  1827 
l:>  1830— it  had  moved  330  feet  downward. 
In  1833  it  had  moved  2354  feet ;  and  in  1841 
M.  Agassiz  found  it  4712  feet  below  its  first 
position. 

140.  In  1840,  M.  Agassiz  himself  and  some 
bold  companions  took  shelter  under  a  great 
overhanging  slab  of  rock  on  the  same  mo- 
raine, to  which  they  added  side-walls  and 
other  means  of  protection.  And  because  he 
wad  his  comrades  came  from  Neufchatel,  the 


hut  was  called  long  afterward  the  "  Hotel 
des  Neuch&telois."  Two  years  subsequent 
to  its  erection  M.  Agassiz  found  that  tke 
"  hotel  "  had  moved  486  feet  downward. 

§  20.  PRECISE  MEASUREMENTS  OF  AGASSIZ 
AND  FORBES. — MOTION  OF  A  GLACIER 
PROVED  TO  RESEMBLE  THE  MOTION  OF  A 
RIVER. 

150.  We  now  approach  an  epoch  in  the 
scientific  history  of  glaciers.     Had  the  first 
observers  been  practically  acquainted  with 
the  instruments  of  precision  used  in  survey- 
ing, accurate  measurements  of  the  motion  of 
glaciers  would  probably  have  been    earlier 
executed.     We  are  now  on  the  point  of  see- 
ing such   instruments  introduced  almost  si- 
multaneously by  M.  Agassiz  on  the  glacier 
of  the  Unteraar,  and  by  Professor  Forbes  on 
the  Mer  de  Glace.     Attempts  had  been  made 
by  M.  Escher  de  la  Linth  to  determine  the 
motion  of  a  series  of  wooden  stakes  driven 
into    the   Ak-tsch    glacier,  but  the    melting 
was  so  rapid  that  the  stakes  soon  fell.     To 
remedy  this,  M.  Agassiz  in  1841  undertook 
the  great  labor  of  currying  boring  tools  to 
his    "  hotel,"  &nd    piercing    the     Unteraar 
glacier  at  six  different  places  to  a  depth  of 
ten  feet,  in  a  straight  line  across  the  glacier. 
Into  the  holes  six  piles  were  so  firmly  driven 
that  they  remained  in  the  glacier  for  a  year, 
ami  in  1842  the  displacements  of  all  six  were 
determined.      They  were    found  to  be    160 
feet,  225  feet,  209"  feet,  245  feet,  210  feet, 
and  125  feet,  respectively. 

151.  A  great  step    is  here  gained       You 
notice  that  the  middle  numbers  ar^  the  larg- 
est.    Tiiey  correspond  to  the  central  portion 
of  the  glacier.     Hence,  these  measurements 
conclusively  establish,  not  only  the  fact  of 
glacier  motion,   but    that   tJie  centre  of  the 
glacier,  like  that  of  a  river,-  moves  more  rapiily 
than  the  xidea. 

152.  With  the  aid  of  trained  engineers  M. 
Agassiz  followed  up  these  measurements  in 
subsequent  years.    His  researches  are  record- 
ed iu  a  work  entitled  "  Systeme  glaciaire," 
which  is  accompanied  by  a  very  noble  atlas 
of  the  Glacier  of  the  Unteraar,  published  in 
1847." 

153.  These  determinations  were  made  by 
means  of  a  theodolite,  of  which  I  will  give 
you  some  notion  feanediately.      The  same 
instrument  was  employed  the  same  year  by 
the  late  Principal  Forbes  upon  the  Mer  de 
Glace.       He  established    independently   the 
greater  central  motion.     He  showed,  more- 
over, that  it  is  not  necessary  to  wait  a  year, 
or  even  a  week  to  determine  the  motion  of  a 
glacier  ;  with  a  correctly-adjusted  theodolite 
he  was  able  to  determine  the  motion  of  vari- 
ous points  of  the  Mer  de  Glace  from  day  to 
day.     He  affirmed,  and  with  truth,  that  the 
motion  of  the  glacier  might  be  determined 
from  hour  to  hour.      We  shall  prove  this 
farther  on  (162).     Professor  Forbes  also  tri- 
angulated the  Mer  de  Glace,  and  laid  down 
an  excellent  map  of  it.     His  first  observa- 
tions and  his  survey  are  recorded  in  a  cele- 
brated book  published  in  1843,  and  entitled 


y  CLOUDS  AND  RIVERS,  ICE  AND  G& 


"Travels  in  tho  Alps." 

154.  These  observations  were  also  followed 
up  in  subsequent  years,  the  results  being  re- 
corded in  a  seties  of  detached  letters  and  es- 
says of  great  interest.  These  were  subse- 
quently collected  in  a  volume  entitled  "  Oc- 
casional Papers  on  the  Theory  of  Glaciers," 
published  in  1859.  The  labors  of  Agassiz 
and  Forbes  are  the  two  chief  sources  of  our 
knowledge  of  glacier  phenomena. 

§  21.  THE  THEODOLITE  AND  ITS  USE.  —  Oui» 
N  MEASUREMENTS. 


155.  My  object  thus  far  is  attained.  i 
have  given  jrou  proofs  of  glacier  motion,  and 
a  historic  account  of  its  measurement.  And 
now  we  must  try  to  add  a  little  to  the  knowl- 
edge of  glaciers  by  our  own  hUws  on  the 
ice.  Resolution  must  not  be  wanting  at  the 
commencement  of  our  work,  ^or  steadfast 
patience  during  its  prosecution.  Look  then 
ac  this  theodolite  ;  it  consists  mainly  of  a 
telescope  and  a  graduated  circle,  the  tele- 
scope capable  of  motion  up  an(i  down,  and 
the  circle,  carrying  the  telescope  along  with 
it,  capable  of  motion  right  and  left.  When 
desired  to  make  the  motion  exceedingly  fine 
an'-l  minute,  suitable  screws,  called  tangent 
screws,  are  employed.  The  instrument  is 
supported  by  three  legs,  movable,  but  firm 
when  properly  planted. 

150.  Two  spirit-levels  are  fixed  at  right 
angles  to  each  other  on  the  circle  just  refei- 
red  to.  Practice  enables  one  to  lake  hold  of 
the  legs  of  the  instrument,  and  so  to  fix  them 
that  tbe  circle  shall  be  nearly  horizontal.  By 
means  of  four  levelling  screws  we  render  it 
accurately  horizontal.  Exactly  under  the 
centre  of  the  instrument  is  a  small  hook  from 
which  a  plummet  is  suspended  ;  the  point  of 
the  bob  just  touches  a  rock  on  which  we 
make  a  mark  ;  or  if  the  earth  be  soft  under- 
neath, we  drive  a  stake  into  it  exactly  under 
the  plummet.  By  re-suspending  the  plum- 
met at  any  future  time  we  can  find  to  a  hat"- 
breadth  the  position  occupied  by  the  instru- 
ment to-day. 

157.  Look  through  the  telescope  ;  you  see 
it  crossed  by  two  fibres  of  the  finest  spider's 
(bread.     In  actual  work  we  first  direct  the 
telescope  across  the  glacier,  until  the  inter- 
section of  the  two  fibres  accurately  covers 
some  well-defined  pcint  of  rock  or  tree  at  the 
other  side  of  the  valley.      This,  our  fixed 
standard,  we  sketch  with  its  surroundings  in 
ft  note-book,  so  as  to  be  able  immediately  to 
recognize  it  oa  our  return  to  this  place.    Irn- 
iginc  a  straight  line  drawn  from  the  centre 
of  the  telescope  to  this  point,  and  that  this 
line  in  permitted  to  drop  straight  down  upon 
the  glacier,   every  point  of  it  falling  as   a 
stone  would  fall  ;  along  such  a  line  we  have 
now  to  fix  a  series  of  stakes. 

158.  A  trained  assistant  is  already  upon 
the  glacier.     He  erects  his  staff  and  stands 
behind  it  ;  the  telescope  is  lowered  without 
swerving  to  the  right  or  to  the  left  ;  in  mathe- 
matical language  it  remains  in  tlie  xame  zerti- 
"Mi  plane.  "  The  crossed  fibres  of  the  tele- 
scope probably  strike  the  ice  a  little   away 


from  the  staff  of  the  assistant  ;  by  a  wave  of 
the  arm  he  moves  right  or  left  ;  he  may 
move  too  much,  so  we  wave  him  back  again. 
After  a  trial  or  two  be  knows  whether  he  is 
near  the  proper  point,  and  if  so  makes  his 
motions  small.  He  soon  exactly  strikes  the 
point  covered  by  the  intersection  of  the 
fibres.  A  signal  is  made  which  tells  him 
that  he  is  right  ;  he  pierces  the  ice  with  an 
auger  and  drives  in  a  stake.  He  then  goes 
forward,  and  in  precisely  the  same  manner 
takes  up  another  point.  After  one  or  two 
stakes  have  been  driven  in,  the  assistant  is 
able  to  take  up  the  other  points  very  rap- 
idly. Any  requisite  number  of  stakes  may 
thus  be  fixed  in  a  straight  line  across  tht> 
glacier. 

159.  Next  morning  we  measure  the  motion 
of  all  the  stakes.     The  theodolite  is  mounted 
in  its  former  position  and  carefully  levelled 
The    telescope   is   directed   first    upon    the 
standard  point  at  the  opposite  side  of  the 
valley,  being  moved  by  a  tangent  screw  until 
the  intersection  of  the  spider's  threads  accu- 
rately covers  the  point.      The  telescope  is 
then  lowered  to  the  first  stake,  beside  which 
our  trained*  assistant    is   already   standing. 
He  is  provided  with  a  staff  with  feet  and 
inches  marked  on  it.     A  glance  shows  us 
that  the  stake  has  moved  down.     By  our  sig- 
nals the  assistant  recovers    the  point  from 
which  we  started  yesterday,  and  then  deter- 
mines the  distance  from  this  point  to  the 
stake.    It  is,  say,  0  inches  ;  through  this  dis- 
tance, therefore,  the  stake  has  moved. 

160.  We  are  careful  to  note  the  hour  anfl 
minute  at  which  each  stake  is  driven  in,  and 
the  hour  and  the  minute  when  its  distance 
from  its  first  position  is  measured  ;  this  ena- 
bles us  to  calculate  the  accurate  daily  motion 
of  the  point   in    question.     The    distances 
through  which  all    the    other    points  have 
moved  are  determined  in  precisely  the  same 
way. 

161.  Thus  we  shall  proceed  to  work,  first 
making  clear  to  our  minds  what  is  to  be 
done,  and  then  making  sure  that  it  shall  be 
accurately  done.     To  give  our  work  reality, 
I  will  here  record  the  actual  measurements 
executed,  and  the  actual  thought  suggested, 
on  the  Mer  de  Glace  in  1857.     The  only  un- 
reality that  I  would  ask  you  to  allow,  Is  that 
you  and  I  are  supposed  to  be  making  the  ob- 
servations together.     The  labor  of  measuring 
was  undertaken  for  the  most  part  by  Mr. 
Hirst. 

§  22.  MOTION  OP  THE  MER  DE  GLACE. 

162.  On  July  14,  then,  we  find  ourselves 
at  the  end  of  the  Glacier  des  Bois,  not  far 
from  the  source  of  the  Arveiron.     We  direct 
our  telescope  across  the  glacier,  and  fix  the 
intersection  of  its  spider's  threads  accurately 
upon  the  edge  of  a  pinnacle  of  ice.      We 
leave    the    instrument    untouched,   looking 
through  it  from  hour  to  hour.     The  edge  of 
ice  moves  slowly,  but  plainly,  past  the  fibres, 
and  at  the  end  of  three  hours  we  assure  our- 
selves that  the  motion  has  amounted  to  sev- 
eral inches.     While  standing  near  the  vault 


THE  FORMS  OF  WATER 


Grancle  Jorasso. 


Col  da 
Geant. 


Chapeau. 


FlG.   6.— OUTLINE-Pl^AN,   SHOWING  THE  MEASURED  LINES  OB1  THE   Ml!U  I)!!   GLACE  AND   ITS  TmBlTTARIBa. 


of  the  Arvoiron,  and  talking  about  going 
into  it,  its  roof  gives  way  and  falls  with  the 
sound  of  thunder.  It  is  not.  therefore,  with- 
out reason  that  I  warned  you  against  enter- 
log  these  vaults  in  summer. 

163.  We  ascend  to  the  Montanvert  Inn, 
fix  on  it  as  a  residence,  and  then  descend  to 
the  lateral  moraine  of  the  glacier  a  little  be- 
low the  inn.  Here  we  erect  our  theodolite, 
and  mark  its  exact  position  by  a  plummet. 
We  must  first  make  sure  that  our  line  is  per- 
pendicular, or  nearly  so,  to  the  axis  or  mid' 
die  line  of  the  glacier.  Our  instructed  assist- 
ant lays  down  a  long  staff  in  the  direction 
of  the  axis,  assuring  himself,  by  looking  up 
and  down,  that  it  is  the  true  direction. 
With  another  staff  in  his  hand,  pointed 


toward  our  theodolite,  he  shifts  his  position 
until  the  second  staff  is  perpendicular  to  the 
first.  Here  he  gives  us  a  signal.  We  direct 
our  telescope  upon  him,  and  then  gradually- 
raising  its  end  in  a  vertical  plane  we  find, 
and  note  by  sketching,  a  standard  point  at  the 
other  side  of  the  glacier.  This  point  known, 
and  our  plummet  mark  known,  we  can  on 
any  future  day  find  our  line.  (To  render  the 
measurements  more  intelligible,  1  append  an 
outline  diagram  of  the  Mer  do  Glace,  and  of 
its  tributaries.) 

164.  Along  the  line  just  described  ten 
stakes  were  set  on  July  I'.th,  1857.  Their  dis- 
placements were  measured  on  the  following 
day.  Two  of  them  had  fallen,  but  here  are 
the  distances  passed  over  by  the  eight  re- 


IN  CLOUDo  AND  RIVERS,  ICE  AND  GLACIERS. 


103 


that  the  glacier  is  retarded  not  only  by  its 
sides  but  by  its  bed  ;  that  the  upper  portions 
of  the  ice  slide  over  the  lower  ones.  Now  if 
Ihe  bed  of  the  Mar  de  Glace  should  have  emi- 
nences here  aud  there  rising  sufficiently  near 
to  the  surface  to  retard  tile  motion  of  the 


malning  ones  in  twenty-four  hours. 
DAILY  MOTION  OF  THE  MER  DE  GLACE. 
FIRST  LINE  :  A  A'  UPON  TUB  SKETCH. 
Eist  West 

Stake 1    2    3    4    5    7    9   10 

Inches 12  17  23  26  25  26  27  33 

165.  You  have  already    assured  yourself   surface,  they  might  produce  the  small  irregu- 
by  actual  contact  that  the  body  of  the  glacier  larities  noticed  above. 

is  real  ice,  and  you  may  have  read  that  169.  We  note  particularly,  wliib  upon  t!ie 
glaciers  move  :  but  the  actual  observation  of  ice,  that  the  26th  stake,  like  the  10th  stake 
the  motion  of  a  body  apparently  so  rigid  is  in  our  last  line,  stands  much  nearer  to  the 
strangely  interesting.  And  not  only  does  the  eastern  than  to  the  western  side  of  the 
fee  move  bodily,  but  one  part  of  it  moves  glacier;  the  ni-.MSiirem«ni  s,  therefore,  off  or 
past  another  ;  the  rate  of  motion  augmenting  a  further  proof  that  the  centre  of  this  portion 
gradually  from  12  inches  a  day  at  the  side  to  of  the  glacier  is  nol  the  place  of  swiftest  mo- 
33  inches  a  day  at  a  distance  from  the  side.  ' 
This  quicker  movement  of  the  central  ice  of 
glaciers  had  been  already  observed  by  Agas- 
siz  and  Forbes  ;  we  verify  their  res-tilts,  and 
now  proceed  to  something  new.  Crossing 
the"  Glacier  du  Geant,  which  occupies  more 
than  half  the  valley,  we  find  that  our  line  of 
stak?s  is  not  yet  at  an  end.  The  10th  stake 
stands  on  the  part  of  the  ice  which  comes 


tion. 

§  23.  UNEQUAL  MOTION  OF  THE  TWO  SIDES 
OF  THE  ME u  DE  GLACE. 


170.  But  in  neither  the  first  line  nor  the 
second  were  we  able  to  push  our  measure- 
across  the  glacier.     Why?    In 
to  do  one  thing    we    are  often 
taught  another,  and  thus  in  science,  if   ,ve 
are  only  steadfast  in  our  wort?,  our  very  de- 
feats are  converted  into  means  of  instruc- 
tion.    We  at  first  planted  our  theodolite  on 
the  lateral  moraine  of  the  Mer  de  Glace,  ex- 

friction  of  the  sides  is  least,  the  motion  ought   £-•£  £$£  £**J^  nowtot- 


from  the  Talrfre. 

106.  Now  the  motion  of  the  sides  is  slow, 
because  of  the  friction  of  the  ice  against  its 
boundaries  ;  but  then  one  would  think  that 
midway  between  the  boundaries,  where  the 


VUSSi.  Si?  SSftS"Mffi  =;  U,e  eentS  ^  ScEp™,  to  U= 

ft5S^^^^.gSrS^tf^«g 

inches  a  da"  81te  S1?e  °*  lll(i  glacier  was  intercepted  by  the 

167.  Here  we  have  something  to  think  of  ;    elevatioa    ^    "»«    c™< re-      T"e 
but  before  a  natural  philosopher  can  think 
with  comfort  he  must  be  perfectly  sure  of 
his  facts.     The  foregoing  line  ran  across  the 


glacier  a  little  below 
Will 


the  Montanvert.     We 


and  to  multiply  our  chances  of  discovery  we 

place  along  it  31  stakes.     On  the  subsequent   which  we  sweep  the 

dav  five  of  these  were  found  unfit  for  IISP  • 


at  the  centre.  The  mountain- 
slopes,  in  fact,  are  warm  in  summer,  ami 
they  melt  the  ice  nearest  to  them,  thus  caus- 
ing a  fall  from  the  centre  to  the  sides. 

171.    But   yonder  on    the    heights  at  the 
other  side  of  the  glacier  we  see  a  likely  place 
for  our  theodolite.     We  cross  the  glacier  and 
plant  our  instrument    in    a    position    from 
glacier  from  side  to 

side.  Our  first  fine  was  below  the  Montan- 
vert,  our  second  line  above  it  ;  this  third  line 
is  exactly  opposite  the  Montanvert ;  in  fact, 
the  mark  on  which  we  have  fixed  the  fibre- 
cross  of  the  theodolite  is  a  corner  of  one  of 
the  windows  of  the  little  mn.  Along  this 
line  we  fix  twelve  stakes  on  July  20th.  On  the 
21st  one  of  them  had  fallen  :  but  the  vi-loci 
ties  of  the  remaining  eleven  in  24  hours  were 
found  to  be  as  follows  : 

THIRD  LINE  :  C  C'  UPON  TUB  SKETCH. 

East  We*t 

Stake, 1      23456789  10    11 

luches 20  23  29  30  34  28  25  23  25  18  9 

172.  Both  the  first  stake  and  the  eleventh 

18th  ;  from  23  inches  at  the  19th  we  fall  to    in  this  series  stor)d  near  the  si(ics  cf  tbe    ,a_ 
from  2o  inches  at  the    -----      ^    -•  -          •  i     .,  ..       .e  — 


remaining  six-and-tvventy  in  24  hours. 
B  B'  UPON  THE  SKETCH. 


SECOND  LINE 
West 

3  4  5 
12  15  15 
16  17  18 
23  23  21 


Stake...  2 
Inches..  11 
b  take...  lo 
Iaches..23 


East 

168.  Look  at  these  numbers.  The  first 
broad  fact  Ihev  reveal  is  the  advance  in  the 
rate  of  motion  from  first  to  last.  There  are, 
however,  small  irregularities  ;  from  2'3  inches 
at  the  17th  stake  we  fall  to  21  inches  at  the 


21  inches  at  the  20th 

21st   we  fall  to  22  inches  at  the  22d  and 

23d  ;   but   notwithstanding  these  small  ups 


cier.  On  the  eastern  side  the  motion  is  20 
inches,  while  on  the  western  side  it  is  only  0. 
It  rises  on  the  eastern  side  from  20  to  34 


and  downs,  the  general  advance  of  the  rate  inchGS  at  lhe  5th  stake  wliicu  we  standillg 

f  motion  is  manifest      Now  there  may  have  upon  thc  glacier  can  see  to  be  much  ucare°r 

been  some  slight  displacement  of  the  stakes  tol  tho  casfern  lh'an  to  the  westem  side.     T/lc. 

by  melting    surhcient  to  account  for  these  unitcd  evidence  of  these  three  lines  places  tl* 

small  deviations  from  uniformity  in  the  in-  factbeyond  doubt,  that  opposite  the  Mwitanvert, 

crease  of  the  motion      But  another  solution  and  for  some  distance  above  it  and  below  it,  tte 

is  also  possible.     We  shall  afterward  learn  ^ole  eastern  side  of  the  glacier  is  mooing  more 


104 


THE  FORMS  OF  WATER 


quickly  than  the  western  side. 

§  24.  SUGGESTION  OP  A  NEW  LIKENESS  OF 
^LACIER  MOTION  TO  RIYEU  MOTION.— 
CONJECTURE  TESTED. 

173.  Here  we  have  cause  for  reflection,  and 
facts  arc  comparatively  worthless  it'  they  do 
not  provoke  this  exercise  of  the  mind.     It  is 
because  facts  of  nature  are  not  isolated  hut 
connected,  th:it  science,  to  follow  tiiem,  must 
also  form  a  connected  whole.     The  mind  of 
the  natural  philosopher  must,  as  it  were,  ba 
ii  web  of  thought  corresponding    in    all  its 
fibres  with  the  web  of  fact  in  nature. 

174.  Let  us,  then   ascend  to  a  point  whi^li 
commands  a  good  view  of  this  portion  of  ihe 
Mer  de  Glace.     The  ice-river  we  see  is  not 
straight  hut,  curved,  and  its  curvature  Isfrotn 
the  Mootauvert ;  that  is  to  say,  its  convex 
side  is  east,  and  its  concave  side  is  west  (look 
to  the  sketch).     You  have  already  pondc;ed 
the  fact  that  a  glacier,  in  gome  respect*,  moves 
like  a  river.       How    would    a    river  move 
through  a  curved  channel  ?    This  is  known. 
Were'tha  ice  of  the  Mer  de  Glace  displaced 
by  water,  the  point  of  swiftest  motion  at  the 
Montanvert  would  not  be  the  centre,  but  a 
point  east  of  the  centre.     Can  it  be  then  that 
this    "water  rock,"    as    ico    is    sometimes 
called,  acts  in  this  respect  also  like  water  ? 

175.  This  is  a  thought  suggested  on  the 
spot ;  it  may  or  it  may  not  be  true,  but  the 
jjieans  <>f  testing  it  are  at  hand.     Looking  up 
the  glacier,  we  see  that  at  ks  Fonts  it  also 
bends,  but  that  there  its  convex  curvature  Is 
toward  the  western  side  of  the  valley  (look 
again  to  the  sketch).     If  our  surmise  be  tiue, 
the  point    of    swiftest    motion    opposite  les 
Fonts  ought  to  lie  west  of  the  axis  of  the 
glacier. 

176.  Ltjt  us  test  this  conjecture.     On  July 
25th  we  fix  in  a  line  across  this  portion  of  the 
glacier  seventeen  stakes  ;  every  one  of  them 
has  remained  firm,  and  on  the  2(jth  we  lind 
the  motion  for  24  hours  to  be  follows  : 

FOURTH  LINE  :  D  D'  UPON  TIIE  SK.ETCU. 

East  West 

Htako 1  2  3  456789  10  11  12  13  14  15 

Inches....?  8  13  15.  16  19  20  ^1  fcl  23  23  sil  22  IT  15 

177.  Inspected  by  the  naked  eye  alone,  the 
stakes  10  and  11,  where  the  clacier  reaches 
its  greatest  motion,  are  seen  to  be  considera- 
bly to  the  west  of  the  axis  of  the  glacier. 
Thus  far  we  have  a  perfect  verification  of 
Wie  guetss  which  prompted  us  to  make  these 
measurements.     You  will  here  observe  that 
the  "  guesses"  of  science  arc  not  the  work 
of  chance,  but  of  thoughtful  pondering  over 
antecedent  farts.     The  guess  is  the  "  induc- 
tion" from  the  facts,  to  be  ratified  or  ex- 
ploded by  the  test  of  subsequent  experiment. 

178.  And  though  even  now  we  have  ex- 
ceedingly strong  reason  for  holding  that  the 
point  of  maximum  velocity  obeys  the  law  of 
liquid  motion,  the  strength  of  our  conclusion 
will  be   doubled  if  we  can    show  that  the 
point  shifts  back  to  the  eastern  side  of  the 
axis  at  another  place  of  flexure.     Fortunate- 
•UT  such  a  place  exists   opposite  Trelapqjte. 


Here  the  convex    curvature  of   tne    vallsy 
turns  again  to  the  east.     Across  this  portioa 
of  the  glacier  a  line  was  set  out  on  -July  28th, 
and  from  measurements  on  the  Ulst,  the  ratu 
of  motion  per  24  hours  was  determined. 
FIFTH  LINTE  :  E  E'  UPON  THIS  SKKTCII.-] 
West  East 

Stake 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15 

Inches..  .11  14  13  15  15  16  17  19  20  19  20  18  16  15  13 

179.  Here,   again,   the   mere   estimate   of 
distances  by  the  oye  would  show  us  that 
the  three  ^ takes  which  moved  fastest,  viz. 
the  9th,  10th,  and  lllli,  were  all  to  the  east 
of  thu  middle  line  of  the  glacier.     The  dem- 
onstration that  the  point  of  swiftest  motion 
wanders  to  and  fro  across  the  axis,  as  the 
flexure  of  the  valley  changes,  is,  therefore 
—shall  I  say  complete  V 

180.  Not  yet.    For  if  surer  menns  are  open 
to  us  we  must  not  rest  content  with  estimates 
by  the  eye.     We  have  with  us  a  surveying 
chain  :  let  us  shake  it  out  and  measure  these 
lines,    noting   the  distance  of  e^ery    stake 
from  the  side  of  the  glacier.     This  is  no  easy 
work  among  the  crevasses,  but  1  confide  it 
confidently  to  Mr.  Hirst  and  you.     \Ve  can 
afterward  compare  a  number  of  .slakes  on  the 
eastern  side  with  the  same  number  of  stakes 
taken  at  the  same  distances  !r-m  the  western 
side.     For  example,  a  pair  of  stakes,  one  ten 
yards  from  the  eastern  side  and  the  other  ten 
yards  fiom  the  western  side;  another  pair, 
one  fifty  yards  from  the  eastern  side  and   the 
other  fifty  yaids  from  the  western  side,  and 
so  on,  can v be  compared  together.     For  the 
sake  of  easy  reference,  let  us  call  the  points 
thus  compared  in  pairs,  eqniraient  points. 

181.  Tin  re  were  five  pairs  of  such  points 
upon  our  fourth  line,   D  1)',   and  here   are 
their  velocities  : 

Eastern  points  ;  motion  in  inches..  13  15  16  18  SO 
\Vertern  points  "  "  -.15  17  1'2  23  23 

In  every  rase  here  the  stake  at  the  western 
side  moved  moie  rapidly  than  tlu  equivalent 
slaKO  at  the  eastern  side. 

182  Applying  the  same  analysis  to  our 
fifth  line,  E  E',  we  have  the  following  series 
of  velocities  of  three  pairs  of  equivalent 
points  : 

Eastern  points  ;  motion  in  inches 15    18    19 

Western  points  "  »•      13    15    17 

183.  Hern  the  three  points  on  the  eastern 
side  move  more  rapidly  than  the  equivalent 
points  on  the  western  side. 

184.  It  is  thus  pioved  : 

1.  That  opposite  the  Montanvert  the  east- 
ern half  of  the  Mer  de  Ulace  moves  more 
rapidly  than  the  western  half. 

2.  That  opposite  lex  Pouts  the  western  half 
of  the  glacier  moves  more  rapidly  than  the 
eastern  half. 

3.  That  opposite  Ti  elaporte  t  .e  eastern  half 
of  the  glacier  again  moves  more  rapidly  than 
the  western  half.  * 

4.  That  these  changes  in  the  place  of  great- 
est motion  are  determined  by  the  flexures  of 
the  valley  through  which  the  Mer  de  Glace 
moves. 


2T  CLOUDS  AND  RIVERS,  ICS  AND  GLACIERS. 


§  25.  NEW  LAW  OP  GLACIER  MOTION. 

185.  Let  u*  express  these  facts  in  another 
va  .  Supposing  the  points  of  swiftest  motion 
m" n  VL-CV  great  number  of  lines  crossing  the 
fler  de  Glace  1o  be  determined  ;  the  line 
oining  all  those  points  together  is  what 
mathematicians  would  call  the  locus  of  the 
3oiut,  of  swiftest  motion. 

ISO.  At  Tielaporte  this  line  would  lie  cast 
)f  the  centre  ;  at  the  Pouts  it  would  lie^vest 
if  the  centre  ;  hence  in  passing  from  Tiela- 
porte to  the  Punts  it  would  cross  the  centre. 
But  at  the  Montanvert  it  would  again  lie 
jast  of  the  centre  ;  hence  between  the  Pouts 
and  the  Mont  an  vert  the  centre  must  be 
crossed  a  second  lime.  If  there  were  further 
•sinuosities  upon  the  Mer  de  Glace  there 
would  he  further  ciossiugs  of  the  axis  of  the 
glacier. 

187.  The  points  on  the  axis  which  mark 
the  transition  from  eastern  to  western  bend- 
ing, and  the  reverse,  may  be  called  jmnts  of 
contrary  flexure. 

188.  Now  what  is  true  of  the  Mer  tie 
Glace  is  true  of  ail  other  glaciers  moving 
through  sinuous  valleys  :  so   that  the  facts 
established  in  the  Mer  (le  Glace  may  be  ex- 
panded into  the    following   general  law  of 
gl  icier  motion  : 

YYiien  a  glacier  moves  through  a  sinuous 
vsiiley.  the  locus  of  the  point  of  maximum 
motion  does  not  coincide  with  the  centre  of 
the  glacier,  but,  on  the  contrary,  always  lies 
on  tue  convex  side  of  the  central  line.  The 
locus  is  therefore  a  curved  line  more  deeply 
sinuous  than  the  valley  itself,  and  crosses 
the  axis  of  the  glacier  at  each  point  of  con- 
trary flexure. 

Is9.  The  dotted  line  on  the  Outline  Plan 
(Fig.  G)  represents  the  locus  of  the  point  of 
maximum  inotbn,  the  iirm  line  marking  the 
centre  of  the  glacier. 

190.  Substituting  t  he  word  river  for  glacier, 
this  law  is  also  true.      The  motion  of  the 
water  is  ruled  by  precisely  the  same  condi- 
tions as  the  motion  of  the  ice. 

191.  Let  us  now  apply  our  law  to  the  ex- 
planation of  a  difficulty.      Turning  to  the 
careful  measurements  executed  by  ftl.  Agas- 
siz  on  the  glacier  of  the  Uuteraar,  we  notice 
in  the  discussion  of  these  measurements  a 
section  of  the  "  Syste"me  glaciaire"  devoted 
to  the  "Migrations  of  the  Centre."     It  is 
here  shown  that  the  middle  of  the  Untcraar 
glacier  is  not  always  the  point  of  swiftest 
motion.      This  fact  has  hitherto   remained 
without  explanation  ;    but  a  glance  at   the 
Unteraar  valley,  or  at  the  map  of  the  valley, 
shows  the  enigma  to  be  an  illustration  of  the 
law  whifh  we  have  just  established  on  the 
Mer  de  Glace. 

^  23.  MOTION  OF  Axis  OP  MER  DE  GLACE. 

192.  We  have  now  measured  the  rate  of 
motion  of  five  different  lines  across  the  trunk 
of  the   Mer  de   Glace.     Do  they  all  move 
alike  ?    No.     Like  a  river,  a  glacier  at  differ- 
ent places  moves   at  different  rates.     Coin- 


paring  together  the  points  of  maximum  mo- 
tion of  all  rive  lines,  we  have  this  result  •. 

MOTION  OF  MER  DE  GLACE. 

At  Treliiporte 20  inches  a  day 

Mies  Punts 

Above  the  Moutanvcrt. 

At  the  Montanvert 

lielow  the  Montanvert 33 

193.  There  is  thus  an  increase  of  rapidity 
as  we  descend  the  glacier  from  Treiaporle  tc 
the  Montanvett  ;  the  maximum  motion  at  the 
Montanvert    being  fourteen  inches    a    day 
greater  than  at  Trelaporte. 

§  27.  MOTION  OP  TRIBUTARY  GLACIERS. 

194.  So  much  for  the  trunk  glacier  ;  lot 
us  now  investigate  the  branches,  permitting, 
as    we    have    hitherto    done,    reflection  OD 
known  facts  to  precede  our  attempts  to  dis^ 
cover  unknown  ones. 

195.  As  we    stood  upon    our  "  cleft  sta- 
tion," whence  we  had  so  capital  a  view  of  the 
Mer  de  Glace,  we  were  struck  by  the  fact  that 
some  of  the  tributaries  of  the  glacier  were 
wider  than  the    glacier    itself.      Supposing 
water  to  be  substituted  for  the  ice,  how  do 
you  suppose  it  would  behave?     You  would 
doubtless  conclude  that  the  motion  down  the 
broad   and    slightly  inclined    valleys  of    the 
Geant  and  the  Lechaud  wouid  be  compara- 
tively slow,  but  that  the  water  would  forca 
itself   with    increased   rapidity  through  the 
"narrows"  of  Tielaporte.     Let  us  test  this 
notion  as  applied  to  ihe  ice. 

19(3.  Planting  our  theodolite  in  the  shadow 
of    Mont  Tacul,' and    choosing   a    suitable 
point  at  the  opposite  side  of  the  Glacier  civ 
Geant,  we  fix  on  July  29th  a  series  of  teE 
stakes  across  the  glacier.     The  motion  of  tlu 
line  in  twenty-four  hours  was  as  follows  : 
MOTION  OF  CLACIER  DU  GEANT. 
SIXTH  LINE:  II II'  UPON  SKETCH. 

Stake 1      *      3      4      5      0      7      8       9    1C 

Indies 11     10    13    1)    1,J    U    ii     10      9     5 

197.  Our  conjecture  is  fully  verified.    The 
maximum  motion  here  is  seven  inches  a  day 
less  than  that  of  the  Mer  dc  Glace  at  Treia- 
porte  (192). 

198.  And  now  for  the   Lechaud   branch. 
On  August  1st  we  lix  ten  stakes  across  this 
glacier  above  the  point  where  it  is  joined  by 
the  Talefre.     Measured  on  August  3d,  and 
reduced  to  twenty-four  hours,  the  motion  was 
found  to  be  : 

MOTION  OP  GLACIER  DE  LECHAUD. 

SEVENTH  LINE:  KK'upo*  SKETCH. 
Stake...           .     1      a      3      4      5      6      7      8      9     1C 
Iaca«  5      8    10      9      J)      S      6      0      7     b 

j09.  Here  our  conjecture  is  still  further 
verified,  the  rate  of  motion  being  even  less 
than  that  of  the  Glacier  du  Geant. 

§  28.  MOTION  OF  TOP  AND  BOTTOM  OF 
GLACIEU. 

200.  We  have  here  the  most  ample  and 
varied  evidence  that  the  sides  of  a  glacier, 
like  those  of  a  river,  are  retarded  by  Jrictioa 
against  its  boundaries.  But  the  likeness  doas 


108 


THE  FORMS  OF  WATER 


not  end  hero.  The  motion  of  a  river  Is  re- 
tarded by  the  friction  against  its  bed.  Two 
observers,  viz.,  Professor  Forbes  and  M. 
Charles  Martins,  concur  in  showing  the  same 
to  be  the  cast?  with  a  glacier.  The  obser- 
vations of  both  have  kjen  objected  to  ;  hence 
it  is  all  the  more  incumbent  on  us  to  seek  for 
decisive  evidence. 

201.  At  the  Tacul  (near  the  point  a  upon 
the  sketch  plan,  Fig.  5)  a  wall  of  ice  about 
150  feet  high  has  already  attracted  our  atten- 
tion. Bending  round  to  joia  the  Lechaud  the 
G lacier  du  Geaut  is  here  drawn  away  from 
the  mountain  side  and  exposes  a  line  section. 
We  try  to  measure  it  top,  bottom,  and  mid- 
dle, and  are  defeated  twice  over.  ^Vu  try  it  a 
third   time  and  succeed.     A.  stake  \j  fixer!  at 
the  summit  of    the    ice-precipice,   another 
at  4  feet  from  the  bottom,  and  a  third  at  35 
feet  above  the  bottom.     These  lower  stakes 
are   tixed   at   some   ri.sk  of  boulders  falling 
upon  us  from  above  ;  but  by  skill  and  CMI- 
tion  we  succeed  in  measuring  the  motions 
of  all  three.     For  2 1  hours  the  motions  aro  : 

Top  *!ake (5     .relics. 

MiJuh!  sU'.;u 4!£ 

B  >;,io.u  3(Ako *'•&      '' 

202.  The  retarding  influence  of  the  bed  of 
the  glacier  is  reduced  to  demonstration  by 
these  measurements.     The  bottom  does  not 
move  with  half  the  velocity  of  the  surface. 

•§  29.  LATERAL  COMPRESSION  OF  A  GLACIER. 

203.  Furnished  with  the  knowledge  which 
those  labors  and  measurements   have  given 
us,  let  us  once  more  climb  to  our  station  be- 
side the  Cleft,  under  the  Aiguille  de  Char- 
moz.     At  our  first  visit  we  saw  the  medial 
moraines  of  the  glacier,  but  we  knew  noth- 
ing about  their  cause.     We  now  know  that 
they  mark  upon  the  trunk  its  tributary  gla- 
ciers.   Cast  your  eye,  then,   first  upon  the 
Glacier  du  Geant ;   realize  its  widtii  in  its 
own  valley,  and  see  how  much  it  is  narrowed 
at  Treiaporte.     The  broad  ice-stream  of  the 
Lechaud     is    still    more    surprising,    being 
squeezed  upon  the  Mer  de  Glace  to  a  narrow 
white  band  between  its  bounding  moraines. 
The  Talefre  undergoes  similar  compression, 
Let  us  now  descend,  shake  out  our  chain, 
measure,  and  express  in  numbers  the  width 
of  the  tributaries,  and  the  actual  amount  of 
compression  suffered  at  Tie'aportc. 

204.  We  find  the  width  of  the  Glacier  du 
G&mt  to  be  5155  links,  or  1134  yards. 

205.  The  width  of  the  Glacier  de  LSchaud 
•we  find  to  be  3725  links,  or  825  yards. 

200.  The  width  of  the  Tatefre  we  find  to 
be  2900  links,  or  638  yards. 

207.  The  sum  of  the  widths  of  the  three 
branch  glaciers  is  therefore  2597  yards. 

208.  At  Treiaporte  these  three  branches 
are  forced  through  a  gorge  893  yards  wide, 
or  one  third  of  their  previous  width,  at  the 
rate  of  twenty  inches  a  day. 

209.  If  we  limit  our  view  to  the  Glacier  dc 
L£chuud,  the  facts  are  still  more  astonishing 
Previous  to  its  junction  with   the   Talefre^ 
this  glacier  has  a  width  of  825  yards  ;   in 
passing  through  the  jaws  of  the  granite  vise 


at  Treiaporte,  its  width  is  reduced  to  eighty- 
eight  yards,  or  in  round  numbers  to  one  tenth 
of  its  previous  width.  (Look  to  the  sketch 
on  page  9.) 

210.  Aro  we  to  understand  by  this  that  the 
ice  of  the  Lechaud  is  squeezed  to  one  tenth 
of  its  former  volume?    By  no  means.     It  is 
mainly  a  change  offonn,  not  of  volume,  Unit 
occurs  at  Treiaporte.     Previous  to  its  com- 
pression, the  glacier  resembles  a  plate  of  ice 
lying  fiat  upon  its  bed.     After  its  compres- 
sion, it  resc-mblcs  a  pl.ue  faced  upon  its  edge. 
The  squeezing,  doubtless,  has  deepened  the 
ice. 

§  CO.  LONGITUDINAL  COMPRESSION  OF  A 
GLACIER. 

211.  The  icj  is  forced  through  the  gorge  at 
Tn'laporte  by  a  piessurc  from  behind;    in 
fact  i  he  Glacier  du  Geant,  immediately  above 
Treiaporte.    represents  a  piston    or    a  plug 
which   drives   the    ice    through    the  gorge. 
What  t'lfect  must  this  pressure  have  upon 
the  plug  itself  V     Reasoning  alone  renders  it 
probable  that  the  pressure  will  shorten  the 
plug  ;  that  the  lower  part  of  the  Glacier  du 
Geaut  will  to  some  extent  yield  to  the  pres- 
sure from  behind. 

212.  Let  us  test  this  notim.     About  three 
quarters  of  a  mile  above  tl-e  Taeul,  and  en 
the  mountain-slope  to  the  lef'L  as  we  ascend. 
we  observe  a  patch  of  verdi/rc.     Thither  wo 
climb  ;  there  we  plant  our  theodolite,  and  set 
out  across  the  Glacier  du  GeV.ut,  aline,  which 
we  will  call  lino  No.  1  (F  F'  upon  sketch, 
Fig.  G.) 

213.  About  a  quarter  of  a  mile  lower  down 
we  find  a  practicable  couloir  on  the  mountain- 
side ;  we  ascend  it,  reach  a  suitable  platform, 
plant  our  instrument,  and  set  out  a  second 
line,  No.  2  (G  G'  upon  sketch).     We  must 
hasten  our  work  here,  for  along  this  couloir 
stones  are  discharged  from  a-  small  glacier 
which  rests  upon  the  slope  of  Mont  Tacul. 

214.  Still  lower  down  by  another  quarter 
of  a  mile,  which  brings  us  ner.r  the  Tacul, 
we  set  out  a  third  line,  No.  3  (II  H'  upon 
sketch),  across  the  glacier. 

215.  The  daily  motion  of  tho  centres  o-f 
these  three  lineaSs  as  follows  : 

Inches.  Distances  asunder. 

No.  1 20-55  i 

No.  2 13-43  f ^  >'ards- 


437 


No.  3 12-75  f- 


216.  The  first  line  here  moves  five  inches 
a  day  more  than  the  second  ;  and  the  second 
nearly  three   inches  a   day  more    than  the 
third.     The  reasoning  is  therefore  confirmed. 
The  ice-plug,  which  is  in  round  numbers  one 
thousand    yards  long,   is  shortened   by  the 
pressure  exerted  on  its  front  at  the  rate  cf 
about  eight  inches  a  day. 

217.  A  river  descending    the  Valley    du 
Geant  would  behave  in  substantially  the  samo 
fashion.      It  would  have  its  motion  on  ap- 
proaching   Treiaporte    diminished,    and    it 
would  pour  through  the  defile  with  a  velocity 
greater  than  that  of  the  water  behind^' 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


H)7 


£  81.  SLIDING  AND  FLOWING.  — HAKD  ICE 
AND  SOFT  ICE. 

218.  We  have  thus  far  confined  ourselves 
to  the  measurement  and  discussion  of  glacier 
motion  ;  but  in  our  excursions  we  have  no- 
ticed many  things  besides.     Here  and  there, 
where  the  ice  has  retreated  fiom  the  moun- 
tain side,  we  have    seen  the   rocks   fluted, 
scored,  and  polished  ;  thus  proving  that  the 
.ice  had  slidden  over  them  and  ground  them 
'down.      At  the  source  of  the  Arveiron  we 
noticed  the  water  rushing  from  beneath  the 
glacier  charged  with  fine  matter.     All  glacier 
rivers  are   similarly  charged.      The  Rhone 
carries  its  load  of  matter  into  the  Lake  of 
Geneva  ;    the  rush  of  the  river  is  here  ar- 
rested, the  matter  subsides,  and  the  Rhone 
quits  the  lake  clear  and  blue.     The  Lake  of 
Geneva,  and  many  other  Swiss  lakes,  are  in 
part  filled  up  with  this  matter,  and  will,  in 
all  probability,  finally  be  obliterated  by  it. 

219.  One  portion  of  the  motion  of  a  glacier 
is  due  to  this  bodily  sliding  of  the  mass  over 
its  bed. 

220.  We  have  seen  in  our  journeys  over 
the  glacier  streams  formed  by  tiie  melting  of 
the  ice,    and   escaping  through  cracks  and 
crevasses  to  the  bed  of  the  glacier.     The  fmc 
matter  ground  down  is  thus  washed  away  ; 
the  bed  is  kept  lubricated,  and  the  sliding  of 
the  ice  rendered  more  easy  than  it   would 
otherwise  be. 

221.  As  a  skater  also  you  know  how  much 
ice  is  weakened  by  a  thaw.     Before  it  actu- 
ally melts   it   becomes    rotten    and  unsafe 
Test  such  ice  with  your  penknife  :  you  cau 
dig  the  blade  readily  into  if,  or  cut  the  ice 
with  ease      Try  good  sound  ice  in  the  same 
way  :  you  find  it  much  more  resistant.     The 
one,  indeed,  resembles  soft  chalk  ;  the  other 
hard  stone. 

222.  Now  the  ]\Icr  ib  Glace  in  summer  is 
in  this  thawing  condition.    Its  ice  is  rendered 
soft  and  yielding  by  the  sun  ;  its  motion  is 
thereby  facilitated.     AVe  have  seen  that  not 
only  docs  the  glacier  slide  over  its  bed,  but 
that  the  upper  layers  slide  over  the  under 
ones,  and  that  the  centre  slides  past  the  sides. 
The  softer  and  more  yielding  the  ice  is,  the 
more  free  will  be  this  motion,  and  the  more 
readily  also  will  it  be  forced  through  a  defile 
like  Trelaporle. 

223.  But  in  winter  the  thaw  ceases  ;  the 
quantity  of  water  reaching  the  bed  of  the 
glacier  is  diminished  or  entirely  cut  off.     The 
ice  also,  to  a  certain  depth  at  least,  is  frozen 
hard.     These  considerations  would  justify 
the  opinion  that  in  winter  the  glacier,  if  it 
moves  at  all,  must  move  more  slowly  than  in 
summer.     At  all  events,  the  summer  meas- 
urements give  no  clue  to  the  winter  motion. 

221.  This  point  merits  examination.  I  will 
not,  however,  ask  you  to  visit  the  Alps  in 
midwinter  ;  but,  if  you  allow  me,  I  will  be 
your  deputy  to  the  mountains,  and  report  to 
you  faithfully  the  aspect  of  the  region  and 
the  behavior  of  the  ice. 


§  32.  WINTER  ON  THE  MER  DE  GLACB. 

225.  The  winter  chosen  is  an    inclement 
one.      There  is  snow  in  London,  snow  ia 
Paris,  snow  in  Geneva  ;  snow  near  Champuni 
so  deep  that  the   road   fences  are  entirely 
effaced.    On  Christmas  night — nearly  at  mid- 
night— 1859,  your  deputy  reaches  Chamouni. 

226.  The  snow  fell  heavily  on  December 
20th  ;  but  on  the  2?th,  during  a  lull  in  the 
storm,  we  turn  out.     There  are  with  me  four 
good  guides  and  a  porter.      They  tie  planks 
to  their  feet  to  prevent  them  from  sinking  iu 
the  snow  ;  I  neglect  this  precaution  and  sink 
often  to  the  waist.     Four  or  five  times  during 
our  ascent  the  slope  cracks  with  an  explosive 
sound,  aud  the  snow  threatens  to  come  down 
in  avalanches. 

The  freshly-fallen  snow  \ /as  in  that  partic- 
ular condition  which  causes  its  granules  to 
adhere,  and  hence  every  flake  fulling  on  the 
trees  had  been  retained  there.  The  laden 
pines  presented  beautiful  and  often  fantastic 
forms. 

227.  After  five  hours  and  a  half  of  arduous 
work  the  Montanvert  was  attained.     We  un- 
locked the  forsaken  auberge.  round  which 
the  snow  was  reared  in  buttresses.     I  have 
already  spoken  of  the  complex  play  of  crys- 
tallizing forces.      The  frost-figures   on  thu 
window-panes  of  the  auberge  were  wonder- 
ful :  mimic  shrubs  and  ferns  wrought  by  the 
building  power  while  hampered  by  the  ad- 
hesion  between   the  glass   and   the  film   in 
which  it  worked.      The  appearance  of  the 
glacier  was  very  impressive  ;  all  sounds  were 
stilled.     The  cascades  which  in  summer  fill 
the  air  with  their  music  were  silent,  hanging 
from  the  ledges  of  the  rocks  in  fluted  col- 
umns of  ice.     The  surface  of  the  glacier  was 
obviously  higher  than  it  had  been  in  sum- 
mer ;  suggesting  tho  thought  that  while  the 
winter  cold  maintained  the  lower  end  of  the 
glacier  jammed  between  its  boundaries,  the 
upper  portions  still  moved  downward  and 
thickened  the  ice.     The  peak  of  the  Aiguille 
da  Dru  shook  out  a  cloud  banner,  the  origin 
and  nature  of  which  have  been  already  ex- 
plained (84). 

228.  On  the  morning  of  the  28th  this  ban- 
ner was  strikingly  large  and  grand,  and  red- 
dened by  the  light  of  tne  rising  sun  it  glowed 
like  a  flame.     Roses  of  cloud  also  clustered 
round  the  crests  of  the  Grande  Jorasse  and 
hung  upon  the  pinnacles  of  Charmoz.     Four 
men,  well  roped  together,  descended  to  the 
glacier.     I  had  trained  one  of  them  in  1857, 
and  he  was  now  to  fix  the  stakes.      The 
storm  had  so  distributed  the  snow  as  to  leave 
alternate  longths  of    the  glacier  bare  anil 
thickly  covered.     Whore  much  snow  lay, 
great  caution  was  required,  for  hidden  cre- 
vasses were  underneath.     The  men  sounded 
with  their  staffs  at  every  step.     Once  while 
looking  at  the  party  through  my  telescope 
the  leader  suddenly  disappeared  ;   the   roof 
of  a  crevasse  had  given  way  beneath  him  : 
but  the  other  three  men  promptly  gathered 
round  and  lifted  him  out  of  the  fissure.     The 
true  line  was  soon  picked  up  by  the  thoodo 


108 


THE  FORMS  OF  WATER 


lite  ;  one  by  one  the  stakes  were  fixed  until 
a  series  of  eleven  of  them  stood  across  the 
glacier. 

229.  To  get  higher  up  the  valley  was  im- 
practicable ;  the  snow  was  too  deep,  and  the 
aspect  of  th'3  weather  too  threatening  ;  so  the 
theodolite  was  planted  amid  the  pines  a  little 
way  below  the  Montanvert,  whence  through 
a  vista  I  could  see  across  the  glacier.  _  The 
men  were  wrapped  at  intervals  by  whirling 
snow-wreaths,  which  quite  hid  them,  and  we 
had  to  take  advantage  of  the  lulls  in  the  wind. 
Fitfully  it  came  up  the  valley,  darkening  the 
air,  catching  the  snow  upon  the  glacier,  and 
tossing  it  throughout  its  entire  length  into 
high  and  violently  agitated  clouds,  separated 
from  each  other  by  cloudless  spaces  corre- 
sponding to  the  naked  portions  of  the  ice.  In 
the  midst  of  this  turmoil  the  men  continued 
to  work.     Bravely  and  steadfastly  stake  after 
stake  was  set  until  at  length  a  series  of  ten 
of  them  was  fixed  across  the  glacier. 

230.  Many  of  the  stakes  were  fixed  in  the 
snow.     They  were  four  feet  in  length,  and 
were  driven  in  to  a  depth  of  about  three 
feet.     But  that  night,  while  listening  to  the 
wild  onset  of  the  storm,  I  thought  it  possible 
that  the  stakes  and   the  snow  which  held 
them  might  be  carried  bodily  away  before 
the  morning.      The  wind,  however,  lulled. 
We  rose  with  the  dawn,  but  the  air  wTas  thick 
with  descending  snow.     It  was  all  composed 
of  those  exquisite  six-petalled  flowers,  or  six- 
rayed  stars,  which  have  been  already  figured 
and  described  (§  9).     The  weather  brighten- 
ing, the  theodolite  was  planted  at  the  end  of 
the  first  line.      The  men  descended,   and, 
trained  by  their  previous  experience,  rapidly 
executed  the  measurements.     The  first  line 
was  completed  before  11   A.M.     Again  the 
snow  began  to  fall,  filling  all  the  air.     Span- 
gles innumerable  were  showered   upon  the 
heights.     Contrary  to  expectation,  the  men 
could  be    seen    and   directed   through   the 
shower. 

231.  To  reach  the  position  occupied  by  the 
theodolite  at  the  end  of  our  second  line,  I  had 
to   wade  breast-deep    through  snow   which 
seemed  as  dry  and  soft  as  flour.     The  toil  of 
the  men  upon  the  glacier  in  breaking  through 
the  snow  was  prodigious.     But  they  did  not 
flinch,  and  after  a  time  the  leader  stood  be- 
hind the  farther  stake,  and  cried,  Nous  awns 
fini.     I  was  surprised  to  hear  him  so  dis- 
tinctly, for  falling  snow  had  been  thought 
very  deadening  to  sound.     The  work  was 
finished,  and  I  struck  my  theodolite  with  the 
feeling  of  a  general  who  had  won   a  small 
Battle. 

28:3.  We  put  the  house  in  order,  packed 
up,  and  shot  by  glissade  down  the  steep 
slopes  of  JM  Film  to  the  vault  of  the  Arvei- 
ron.  We  found  the  river  feeble,  but  not 
dried  up.  Many  weeks  must  have  elapsed 
since  any  water  had  been  sent  down  from  the 
surface  of  the  glacier.  But  at  the  setting  in 
of  winter  the  fissures  were  in  a  great  measure 
charged  with  water  ;  and  the  Arveiron  of 
to-day  was  probably  due  to  the  gradual 
drainage  of  the  glacier.  There  was  now  no 


danger  of  entering  the  vault,  for  the  ice 
seemed  as  firm  as  marble.  In  the  cavern  we 
were  bathed  by  blue  light.  The  strange 
beauty  of  the  place  suggested  magic,  and  put 
me  in  mind  of  stories  about  fairy  caves  which 
I  had  read  when  a  boy.  At  the  source  of  the 
Arveirou  our  winter  visit  to  the  Mer  de  Glace 
ends  ;  next  morning  your  deputy  was  on  his 
way  to  London. 

§  33.   WINTER  MOTION  OF  THE  Msii  DE 
GLACE. 

233    Here  are  the  measurements  executed 
in  the  winter  of  1859  : 

LINE  No.  I. 

Stake t     2      .3      4      5      6      7      8      9     10    11 

Inches 7    11    It    13    14    14    10    13    12    13     7 

LINB  No.  IT. 

Stake 1     2      3      4      5      0      7      8      9     10 

Inches     8    10    14    Iti    16    1J    13    17    15    14 

234.  Thus  the  winter  motion  of  the  Mer 
de  Glace  near  the  Montanvert  is,  in  round 
numbers,  half  the  summer  motion. 

235.  As  in  summer,  the  eastern  side  of  the 
glacier  at  this  place  moved  quicker  than  the 
western. 

§34.  MOTION   OF  THE  GHINDELWALD  AND 
ALETSCII  GLACIEKS. 

236.  As  regards  the  question  of  motion, 
to  no  other  gfacier  have  WTC  dtvoted  ourselves 
with  such  thoroughness  as  to   the  Mer  de 
Glace  ;  we  are,  however,  abie  to  add  a  few 
measurements  of  other  celebrated  glaciers. 
Near  the  village  of  Gimdelwalcl  in  the  Ber- 
nese   Oberland,    there  are   two    great    ice- 
streams  called  lespectively  the  Upper  and  the 
Lower  Grindelvvald  glaciers,  the  second  of 
which  is  frequently  visited  by  travellers  in 
the  Alps.     Across  it  on  August  6th,  1800,  a 
series  of  twelve    stakes  was  fixed  by  Mr. 
Vaughan  Hawkins  and  myself.      Measured 
on  the  8th  and  reduced  to  its  daily  rate,  the 
motion  of  these  stakes  was  as  follows  : 

MOTION  OF  LOWER  GRINDELWALD  GLACIER. 

East  West 

Stake...  1      23  5      6      7      8      9     19    11    12 

Inches..  18    11)    20    21    21    21    22    20    19    18    17    14 

237.  The   theodolite    was  here  planted  a 
little  below  the  footway  leading  to  the  higher 
glacier  region,  and  at  about  a  mile  above  the 
end  of  the  glacier.     The  measurement  was 
rendered  difficult  by  crevasses. 

238.  The  Ingest  glacier  in  Switzerland  is 
the  Great  Aletsch,  to  which  further  reference 
shall  subsequently  be  made.     Across  it  on 
August  14th,  1860,  a    series  of  thirty-four 
stakes  was  planted  by  Mr.  Hawkins  and  me. 
Measured  on  the  16th  and  reduced  to  their 
daily  rate,  the  velocities  were  found  to  be  as 
follows  : 

MOTION  OF  GREAT  ALETSCH  GLACIER. 
East 

Stake 12345      6      7      8      9     10    11  13 

Inches 23468     11    13    14    18    17    17  W 

Stake 13    14    15    16    17    13    19    20    21    21  S3 

Inches 19    18    18    17    19    19    19    19    17    17  13 

Stake 24    25    a<i    27    28    29    80    31    83    33  31 

Inches 16    17    K    17    17    17    17    17    16    12  13 

West 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


109 


239.  The  maximum  motion  here  is  nine- 
teen inches  a  day.  Probably  the  eastern  side 
of  the  glacier  is  shallow,  the  retardation  of 
the  betf  making  the  motion  of  the  eastern 
slakes  inconsiderable.  The  width  of  the 
glacier  here  is  1)030  links,  or  about  a  mile 
and  a  furlong.  The  theodolite  was  planted 
high  among  ^the  rocks  on  the  western  flank 
ot'  the  mountain,  about  half  a  mile  above  the 
Margelin  See. 

§  35.  MOTION  OF  MORTERATSCII  GLACIER. 
240  Far  to  the  east  of  the  Oberland,  and 
in  that  interesting  part  of  Switzerland  known 
as  the  Ober  Eugadin,  stands  a  noble  group  of 
mountains,  less  in  height  than  those  of  the 
Oberland,  but  still  of  commanding  elevation. 
The  group  derives  its  name  from  its  most 
dominant  peak,  the  Piz  Bernina.  To  reach 
the  place  we  travel  by  railway  from  Basel  to 
ZUrich,  and  from  Zurich  to  Chur  (French 
Coire),  whence  we  pass  by  diligence  over 
either  the  Albula  pass  or  the  Juliar  pass  to 
the  village  of  Pontresina.  Here  we  are  in 
the  immediate  neighborhood  of  the  Bernina 
mountains. 

241.  From  Pontresina   we   may   walk  or 
drive  along  a  good  coach  road  over  the  Ber- 
nina pass  into  Italy.     At  about  an  hour  above 
the  village  you  would  look  from  the  road 
into  the  heart  of  the  mountains,  the  line  of 
vision  passing  through  a  valley,  in  which  is 
couched    a    glacier    of    considerable    size. 
Along  its  back  you  would  trace  a  medial 
moraine,  and  you  could  hardly  fail  to  notice 
how  the  moraine,  from  a  mere  narrow  streak 
at  first,  widens  gradually  as  it  descends,  un- 
til finally  it  quite  covers  the  lower  end  of  the 
glacier.     Nor  is  this  an  effect  of  perspective  ; 
lor  were  you  to  stand  upon  the  mountain 
slopes  which  nourish  the  glacier,  you  would 
see  thence  also  the  widening  of  the  streak  of 
rubbish,  though  the  perspective  here  would 
tend  to  narrow  the  moraine  as  it  retreats 
downward. 

242.  The  ice-stream  here  referred  to  is  tho 
Morteratsch  glacier,  the  end  of  which  is  a 
short  hour's  walk  from  the  village  of  Pon- 
tresina.    We  have  now  to  determine  its  rate 
of  motion  and  to  account  for  the  widening  of 
its  medial  moraine. 

243.  In  the  summer  of  18G4  Mr.  Hirst  and 
myself  set  out  three  lines  of  stakes  across  the 
glacier.     The  first  line  crossed  the  ice  high 
up  ;  the  second  a  good  distance  lower  down, 
and  t.he  third  lower  still.     Even  thrj  third 
line,  however,  was  at  a  considerable  distance 
above  the  actual  snout  of  the  glacier.     Tho 
daily  motion  of  these  three  lines  was  as  fol 
lows  : 

FIUST  LINE. 

Stake 1     2      3      4      5      6      7      8      9     10    11 

Inches 8    12    13    13    14    13    12    !„»    1J      7      5 

SECOND  LINE. 

Stake.   1     2      3      4      5      6      7      8      9     10    11 

inches 1     4      6      8     10    11     11     11    11     H    n 

TIIIRD  LINE. 

Stake 1     2      3      4      5      G      7      8      9     n    11 

Inches 1     24      5V    607755       4 

244.  Compare  these  lines  together.     You 


notice  the  velocity  of  the  first  is  greater  than 
that  of  the  second,  and  the  velocity  of  tho 
second  greater  than  that  of  the  third. 

245.  The  lines  were  permitted  to  move 
downward    for    100  hours,    at  the  end   of 
which  lime  the  spaces  passed  over  by  the 
points  of  swiftest  motion  of  the  three  lines 
were  as  follows  : 

MAXIMUM  MOTION  IN  100  HOURS. 

First  line 50  inches. 

Second  line  4-4       " 

Third  line 30      " 

246.  Here  then  is  a  demonstration  that  the 
upper  portions  of  the  Morteralsch  glacier  are 
advancing  on  the  lower  ones.     In  1871  tJie 
motion  of  a  point  on  tlie  middle  of  the  glacier 
near  its  snout  was  found  to  be  less  than  two 
inches  a,  day  ! 

247.  What,  then,  is  the   consequence  of 
this  swifter  march  of  the  upper  glacier? 
Obviously   to  squeeze   this  medial  incline 
longitudinally,  and  to  cause  it  to  spread  out 
laterally.     We  have  here  distinctly  revealed 
the  cause  of  the  widening  of  the  medial  mo- 
raine. 

248.  It  has  been  a  question  much  dis- 
cussed, whether  a  glacier  is  competent  to 
sc'oop  out  or  deepen  the  valley  through  which 
it  moves,  and  this  very  Morteratsch  glacier 
has  been  cited  to  prove  that  such  is  not  the 
case.     Observers  went  to  the  snout  of  the^ 
glacier,  and    finding  it  sensibly  quiescent, 
they  concluded  that  no  scooping  occurred*. 
But  those  who  contended  for  the  power  01 
glaciers  to  excavate  valleys  never  stated,  or 
meant  to  state,  that  it  was  the  snout  of  the* 
glacier  which  did  the  work.     In  the  Morte- 
ratsch glacier  the  work  of  excavation,  which* 
certainty  goes  on  to  a  greater  or  less  extent*, 
must  be  far  more  effectual  high  up  the  val-, 
ley  than  at  the  end  of  the  glacier. 

§  36.    BIRTH   OP   A   CREVASSE  :    REFLEO 

TIONS. 

240.  Preserving  the  notion  that  we  are- 
working  together,  we  will  now  enter  upon  *k 
new  field  of  inquiry.  We  have  wrapped  up 
our  chain  and  are  turning  homeward  after 
a  hard  day's  work  upon  the  Glacier  du 
Geant,  when  under  our  feet,  as  if  coming 
from  the  body  of  the  glacier,  an  explosion  is 
heard.  Somewhat  startled,  we  look  inquir- 
ingly over  the  ice.  The  sound  is  repeated,, 
several  shots  being  fired  in  quick  succession. 
They  seem  sometimes  to  our  right,  some- 
times to  our  left,  giving  the  impression  that 
the  glacier  is  breaking  all  round  us.  Still 
nothing  is  to  be  seen. 

250.  We  closely  scan  the  ice,  and  after  an. 
hour's  strict  search  we  discover  the  cause  of 
the  reports.  They  announce  the  birth  of  a 
crevasse.  Through  a  pool  upon  the  glacier 
we  notice  air-bubbles  ascending,  and  find 
the  bottom  of  the  pool  crossedtoy  a  narrow 
crack,  from  which  the  bubbles  issue.  Right 
and  left  from  this  pool  we  trace  the  young 
fissure  through  long  distances.  It  is  some- 
times almost  too  feeble  to  be  seen,  and  at  no 
Clace  is  it  wide  enough  to  admit  a  knite- 
lade. 


THE  FORMS  OF  TVATEP 


251.  It  is  difficult  to  believe  that  the  for. 
midable  fissures,  among  which  you  and  I 
havu  so  often  trodden  with  awe,  could  com- 
mence in  this  small  way.  Such,  however,  is 
the  case.  The  great  and  gaping  chasms  on 
and  above  the  ice-fails  of  the  Geaut  and  the 
Talcfre  begin  as  narrow  cracks,  which  open 
gradually  to  crevasses.  We  are  thus  taught 
in  an  instructive  and  impressive  way  that  up 
pearances  suggestive  of  very  violent  action 
may  really  be  produced  by  processes  so  slow 
as  to  require  refined  observations  l  >  detect 
them.  In  the  production  of  natural  phe- 
nomena two  things  always  conic  into  play, 
the  inteiixity  of  Ihe  acting  force,  and  I  he  time 
during  which  it  acts.  Make  the  intensity 
great  and  the  time  small,  an  1  you  have  sud- 
den convulsion  ;  but  precisely  the  same  ap- 
parent effect  may  be  produced  by  making  the 
intensity  small  and  the  time  great.  This 
truth  is  strikingly  illustrated  by  the  Alpine 
icv-falls  and  crevasses  ;  and  many  geological 
phenomena,  which  at  first  sight  suggest  vio- 
lent convulsion,  may  be  really  produced  in 
the  self-same  almost  impercepiibb  way. 

§  37.  ICICLES. 

2/52.  The  crevasses  are  grandest  on  the 
higher  neves,  where  they  sometimes  appear 
as  long  yawning  fissures,  and  sometimes  as 
chasms  of  irregular  outline.  A  delicate  blue 
light  shimmers  from  them,  but  this  is  grad- 
ual ty  lost  in  the  darkness  of  their  profounder 
portions.  Over  the  edges  of  the  chasms, 
and  mostly  over  the  southern  edges,  hangs  a 
coping  of  snow,  and  from  this  depend  like 
stalactites  rows  of  transparent  icicles,  10,  20, 
80  feet  long.  These  pendent  spears  consti- 
tute one  of  the  most  beautiful  features  of  the 
higher  crevasses. 

253.  How  are  they  produced  ?    Evidently 
by   the   thawing  of    the  snow.     But   why, 
Avhen  once  thawed,  should  the  water  freeze 
again  to  solid  spears  ?    You  have  seen  icicles 
pendent  from  a  house-eave,  which  have  been 
manifestly  produced  by  the  thawing  of  the 
snow  upon  the  roof.    If  we  understand  these 
we  shall  also  understand  the  vaster  stalactites 
of  the  Alpine  crevasses. 

254.  Gathering  up  such  knowledge  as  we 
possess,  and  reflecting  npuii  it  patiently,  let 
us  found  on  it,  if  we  can,  a  theory  of  icicles 

255.  First,  then,  you  aie  to  know  that  the 
air  of  our  atmosphere  is  hardly  heated  at  all 
by  the  rays  of  the  sun,  whether  visible  or  in- 
visible.    The  air  is  highly  transparent  to  all 
kinds  of  rays,  and  it  is  only  the  scanty  frac- 
tion to  which  it  is  not  transput  cut  that  ex- 
pend their  force  in  warming  it. 

256.  Not  so,  however,  with  the  snow  on 
which  the  sunbeams  fall.     It  absorbs   the 
solar  heat,  and  on  a  sunny  day  you  may  see 
the  summits  of  the  high  Alps  glistening  with 
the  water  of  liquefaction.     The  air  above 
and  around  the  mountains  may  at  the  same 
time  be  many   degrees  below  the  freezing 
point  in  temperature. 

257.  You  have  only  to  pass  from  sunshine 
into   shade    to    prove  this.     A  single   step 
suffices  to  carry  you  from  a  place  where  the 


thermometer  stands  high  to  one  wneie  it 
stands  )ow  ;  the  change  being  due,  not  to 
any  difference  in  the  temperature  of  the  air, 
but  simply  to  the  withdrawal  of  the  ther- 
mometer from  the  direct  action  of  the  solar 
rays.  Nay,  without  shifting  the  thermome- 
ter at  all,  by  interposing  a  suitable  screen, 
which  cuts  off  the  sun's  rays,  the  coldness  of 
the  air  may  be  demonstrated. 

258.  Look  now  to    the  snow  upon  your 
house-roof.      The    sun  plays  upon  it   and 
melts  it  ;  the  water  trickles  to  the  cave  and 
then  drops  down.     If  the  eave  face  the  sun 
the  water  remains  water  ;  but  if  the  eave  do 
not  face  the  sun,  the  drop,  before  it  quits  its 
parent  snow,  is  already  in  shadow.     Now  the 
shaded  space,  as  we  have  learned,  may  be 
below  the  freezing  temperature.     If  so,  the 
drop,  instead  cf  falling,  congeals,  and  the 
rudiment  of    an    icicle    is   formed.     Other 
drops  and  driblets  succeed,   which  trickle 
over  the  rudiment,   congeal  upon  it  in  part 
and  thicken,  it  at  the  root.     But  a  portion  of 
the  water  reaches  the  free  end  of  the  icicle, 
hangs  from  it,  and  is  there  congealed  before 
it  escapes.     The  icicle  is  thus  lengthened.     In 
the  Alps,  where  the  liquefaction  is  copious 
and  the  cold  of  the  shaded  crevasse  intense, 
the  icicles,  though  produced  in  the  same  way. 
naturally  grow  to  a  greater  size.     The  drain- 
age of  the  snow  after  the  sun's  power  is  with- 
drawn also  produces  icicles. 

259.  It  is  interesting  and  important  that 
you  should  be  able  to  explain  the  formation 
of  an  icicle  ;    but  it  is  far  more  important 
that  you  should  realize  the  way  in  which  the 
various  threads  of  what  we  call  Nature  are 
woven  together.     You   cannot  fully  under- 
stand an  icicle  without  first  knowing  that 
solar  beams  powerful  enough  to  fuse  the 
snows  and  blister  the  human  skin,  nay,  it 
might  be  added,  powerful  enough,  when  con- 
centrated, to  burn  up  the  human  body  itself, 
may  pass  through  the  air  and  still  leave  it  at 
an  icy  temperature. 

§  38.  THE  BERGSCHRUND. 

260.  Having  cleared  away  this  difficulty, 
let  us  turn  once  more  to  the  crevasses,  taking 
them  in  the  older  of  their  formation.     First 
then  above  the  neve  we  have  the  final  Alpine 
peaks  and  crests,  against  which  the  snow  is 
often  reared  as  a  steep  buttress.     We  have 
already  learned  that  both  neves  and  glaciers 
are  moving  slowly  downward  ;  but  it  usual- 
ly happens  that  the  attachment  of  the  high- 
est portion  of  tho  buttress  to  the  rocks  is 
great  enough  to  enable  it  to  hold  on  while 
the   lower    portion    breaks  away.     A   very 
characteristic  crevasse  is  thus  formed,  called 
in  the  German-speaking  portion  of  the  Alps 
a  Beryschrund.     It  often  surrounds  a  peak 
like  a  fosse,  as  if  to  defend  it  against  the  as- 
saults of  climbers. 

261.  Look  more  closely  into  its  formation. 
Imagine   the    snow    as*  yet  unbroken.     Its 
higher  portions  cling  to  the  rocks  and  move 
downward  with  extreme  slowness.     But  its 
lower  portions,  whether  from  their  greater 
depth  aud  weight  or  their  less  perfect  at" 


CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


iir 


tacnment,  arc  compelled  to  move  more 
quick?}'.  A.putt\s  therefore  exerted,  tend- 
ing to  separate  the  lower  from  the  upper 
snow.  For  a  lime  this  pull  is  resisted  by  the 
cohesion  of  the  neve  ;  but  this  at  length 
gives  way,  and  a  crack  is  formed  exactly 
across  the  line  in  which  the  pull  is  exerted. 
In  other  words,  «  crevasse  is  for  fried  at  rig/it 
angles  to  the  line  of  tension. 

§  39.  TRANSVERSE  CREVASSES. 

262.  Both  on  the  neve'  and  on  the  glacier 
the   origin  of    the    crevasses    is  the  same. 
Through  some  cause  or  other,    the   ice  is 
thrown  into  a  state  of  strain,  and  as  it  can- 
not stretch  it  breaks  across  the  line  of  tension. 
Take,  for  example,  the  ice-full  of  the  Geant, 
or  of  the  Talefre,  above  which  you  know 
the  ^crevasses  yawn  terribly.      Imagine  the 
neve  and  the  glacier  entirely  peeled  away, 
so  as  to  expose  the  surface  over  which  they 
move.     From  the  Col  du  Geant  we  should 
sue  this  surface  falling  gently  to  the  place 
uo\v  occupied  by  the  brow  of  the  cascade. 
Here  the  surface  would  fall  steeply  down  to 
the  bed   of   the   present   Glacier  du  Ge'ant, 
v/liera  the  slope  would  become  gentle  once 
more. 

26-J.  Think  of  the  neve" moving  over  such 
a  sin  face.  It  descends  from  the  Col  till  it 
reaches  the  brow  just  referred  to,  It  crosses 
the  brow,  and  must  bend  down  to  keep  upon 
its  bed.  Realize  clearly  what  must  occur. 
Tlie  surface  of  the  ne"ve  is  evidently  thrown 
iito  a  state  of  strain  :  it  breaks  and  forms  a 
Crevasse.  Each  fresh  portion  of  the  ne*ve  as 
it  passes  the  brow  is  similarly  broken,  and 
thus  a  succession  of  crevasses  is  sent  down 
the  fall.  Between  every  two  chasms  is  a 
great  transverse  ridge.  Through  local  strains 
upon  the  fall  those  ridges  are  also  frequently 
broken  across,  towers  of  ice — xtfracs — being 
the  result.  Down  the  fall  both  ii<iges  and 
se'racs  are  borne,  the  dislocation  being  aug- 
mented during  the  descent. 

204.  What  must  occur  at  the  foot  of  the 
fall?  Here  the  slope  suddenly  lessens  in 
steepness.  It  is  plain  that  the  crevasses 
must  not  only  cease  to  open  here,  but  that 
they  must  in  whole  or  in  part  close  up.  At 
the  summit  of  the  fall,  the  bending  was  such 
as  to  make  the  surf  ace  con  vex  ;  at  the  bottom, 
of  the  fall,  the  bending  renders  the  surface 
concave.  In  the  one  case  we  have  strain,  in 
the  other  pressure.  In  the  one  case,  therefore, 
we  have  the  opening,  and  in  the  other  the 
closing  of  crevasses.  This  reasoning  corre- 
sponds exactly  with  the  facts  of  observation. 

263.  Lay   bare  your  arm  end   stretch   it 
straight.     Make  two  ink  dots  half  an  inch  or 
an  inch  apart,  exactly  opposite  the  elbow. 
Bend  your    arm,    the  dots  approach   each 
other,  and  are  finally  brought  together.     Let 
the  two  dots  represent  tiie  two  sides  of  a 
crevasse  at  the  bottom  of  an  ice-fall  ;  the 
bending  of  the  arm  resembles  the  bending  of 
the  ice,  and  the  closing  up  of  the  dots  re- 
sembles the  closing  of  the  fissures. 

266.  The  same  remarks  apply  to  various 
portions  of  the  Mer  de  Glace.  At  certain 


places  the  inclination  changes  from  a  gentler 
to  a  steeper  slope,  and  on  crossing  the  brow 
between  both  the  glacier  breaks  its  back. 
Transverse  crevasses  are  thus  formed.  There 
as  such  a  change  of  inclination  opposite  to  the 
Angle,  and  a  still  greater  but  similar  change 
at  the  head  of  the  Glacier  des  Bois.  The 
consequence  is  that  the  Mer  de  Glace  at  the 
former  point  is  impassable,  and  at  the  latter 
the  rending  and  dislocation  are  such  as  we 
have  seen  and  described.  Below  the  Angle, 
and  at  the  bottom  of  the  Glacier  des  Bois, 
the  steepness  relaxes,  the  crevasses  heal  up, 
and  the  glacier  becomes  once  more  continu- 
ous and  compact. 

§  40.  MARGINAL  CREVASSES. 
2G7.  Supposing,  then,  that  we  had  no 
changes  of  inclination,  should  we  have  no 
crevasses  ?  We  should  certainly  have  less  of 
them,  but  they  would  not  wholly  disappear. 
For  other  circumstances  exist  to  throw  the 
ice  into  a  state  of  strain,  and  to  determine  its 
fracture.  The  principal  of  these  is  the  more 
rapid  movement  of  the  centre  of  the  glacier, 

268.  Helped  by  the  labors  of  an  eminent 
man,  now  dead,  the  late  Mr.  Wm.  Hopkins, 
of  Cambridge,  let  us  master  the  explanation 
of  this  point  together.     But  the  pleasure  of 
mastering  it  would  be  enhanced  if  we  could 
see  beforehand  the  perplexing  and  delusive 
appearances  accoun'-jd  for  by  the  explana- 
tion.    Could  my  wishes  be  followed  out,  I 
would  at  this  point  of  our  researches  carry 
you  off  with  me  to  Basel,  thence  to  Thun, 
thence  to  Interlaken,  thence  lo  Grindelwald, 
where  you  would  rind  yourself  in  the  actuat 
presence  of  the  Wetteihorn  arid  the  Eiger, 
with  all  the  greatest  peaks  of  the  Bernese 
Oberland,  the  Finsteraaihorn,  the  Schreck-r 
horn,  the    Mouch,   the  Jungfrau,   at  hand. 
At  Grindelwald,  as  we  have  already  learned, 
there  are  two  well  known  glaciers — theObe? 
Grindelwald  and  the  Unter  Grindelwald  gla- 
ciers— on  tlu  latter  of  which  our  observa- 
tions should  commence. 

269.  Dropping  down  from  the  village  tQ 
the  bottom  of  the  valley,  we  should  breast 
the  opposite  mountain,  and  with  the 'great 
limestone  precipices  of    the  Wetterhorn  to 
our  left,  we  should  get  upon  a  path  which 
commands  a  view  of  the  glacier.     Here  we 
should  see  beautiful  examples  of  the  opening 
of  crevasses  at  the  summit  of  a  brow,  and 
their  closing  at  the  bottom.     But  the  chief 
point  of    interest   would    be   the   crevasses 
formed  at  the  side  of  this  glacier — the  mar- 
ginal crevasses,  as  they  may  be  called. 

270.  We  should  find  the  side  copiously  fis^ 
sured,  even  at  those  places  where  the  centre 
is  compact  ;  and  we  should  particularly  no- 
tice that  the  lissures  would  neither  run  in 
the  direction  of    the    glacier    nor  straight 
across  it,  but  that  they  would  be  oblique  to  it, 
inclosing  an  angle  of  about  45  degrees  with 
the  sides.     Starling  from  the  side  of  the  gla- 
cier the  crevasses  Avould  be  seen  to  point  up- 
ward ;    that  is  to  say,  the  ends  of  the  fis- 
sures abutting  against  the  bounding  moun- 
tain would  appear  to  be  drrjgtd  down.     Were 


113 


THE  FORMS  OF  WATER 


L                        S                          5?                                    C 

— 

m 

B                    T                       T                                      D 

FIG.  7. 

you  less  instructed  than  you  now  are,  I 
might  lay  a  wager  that  the  aspect  of  these 
fissures  would  cause  you  to  conclude  that  the 
centre  of  the  glacier  is  left  behind  by  the 
quicker  motion  of  the  sides. 

271.  This    indeed     was    the    conclusion 
drawn  by  M.  Agassiz  from  this  very  appear- 
ance, before  he  had  measured  the  motion  of 
the  sides  and  centre  of  the  glacier  of  the 
Unteraar.     Intimately  versed  with  the  treat- 
ment of  mechanical  problems,  Mr.  Hopkins 
immediately  deduced   the  obliquity  of   the 
lateral  crevasses  from  the  quicker  How  of  the 
centre.     Standing  beside  the    glacier    with 
pencil  and  note-book  in  hand,"  I  would  at 
once  make  the  matter  clear  to  you  thus. 

272.  Let  A  c,  in  the  annexed  figure,  be  one 
side  of  Ihe  glacier,  and  B  D  the  other  ;  and 
let  the  direction  of  motion  be  that  indicated 
t>y  the  arrow.     Let  s  T  be  a  transverse  slice 
of  the  glacier,  taken  straight  across  it,  say  to- 
day.    A  few  days  or  weeks  hence  this  slice 
wi'll  have  been  carried -down,  and  because  the 
centre  moves  more  quickly  than  the  bides  it 
will  not  remain  straight,  but  will  bend  into 
the  form  s'  T'. 

273.  Supposing  T  i  to  be  a  small  square  of 
the  original  slice  near  the  side  of  the  glacier. 
In  its  new  position  the  square  will  be  distort- 
ed to  the  lozenge-shaped  figure  T'  i'.     Fix 
your  attention  upon  the  diagonal  T  i  of  the 
square  :  in  the  lower  position  this  diagonal, 
if  the  ice  could  stretch,  would  be  lengthened 
to  T'  i'.     But  the  ice  does  not  stretch  ;  it 
breaks,  and  we  have  a  crevasse  formed  at 
right  angles  to  T'  i.     The  mere  inspection  of 
the  diagram  will  assure  you  that  the  crevasse 
will  point  obliquely  upward. 

274.  Along  the  whole  side  of  the  glacier 
the  quicker  movement  of  the  centre  produces 
a  similar   state   of  strain  ;    and    the  conse- 
quence is  that  the  sides  are  copiously  cut  by 
those  oblique  crevasses,  even  at  places  where 
the  centre  is  free  from.  them. 

275.  It  is  curious  to  see  at  other  places  the 
transverse  fissures  of  the  centre  uniting  with 
those  at  the  sides,  so  as  to  form  great  curved 
crevasses  which    stretch   across  the  glacier 
A'rom  side  to  side.     The  convexity  of  the 
curve  is  turned  upward,  as  mechanical  prin- 
ciples declare  it  ought  to  be.     But  if  you 
were  ignorant  of  those  principles,  you  would 
never  infer  frorn  the  aspect  of  these  curves 
the  quicker  motion  of  the  centre.     In  land- 
slips, and  in  the  motion  of  parliaily  indurat- 
ed mud,  you  may  sometimes  notice  appear- 
ances similar  to  those  exhibited  by  the  ice. 


§  41.  LONGITUDINAL  CREVASSES 

276.  "We  have  thus  unravelled  the  orlgia 
of  both  transverse  and  marginal  creva-ses. 
But  where  a  glacier  issues  from  a  steep  and 
narrow   defile  upon   a  comparatively    level 
plain  which  allows  it  room  to  expand  later- 
ally, its  motion  is  in  part  arrested,  and  the 
level  portion  has  to  bear  the  thrust  of  the 
steeper  portions  behind.     Here  the  line  of 
thrust  is  in   the   direction  of    the  glacier, 
while  the  direction  at  right  angles  to  this  is 
one  of  tension.     Across  this  latter  the  glacier 
breaks,  and  longitudinal  crevasses  are  formed. 

277.  Examples  of  this  kind  of  crevasse  are 
furnished  by  the  lower  pait  of  the  Glacier  of 
the  Rhone,  when  looked  down  upon  from 
the  Gi  imsel  Pass,  or  from  any  commanding 
point  on  the  flanking  mountains. 

§  42.  CREVASSES  IN  RELATION  TO  CUIIVA- 
TUKE  OP  GLACIER. 

278.  One  point  in  addition  remains  to  bo 
discussed,  and  your  present  knowledge  will 
enable  you  to  master  it  in  a  moment.     You 
remember  at  an  early  period  of  our  researches 
that  we  crossed  the  Mer  de  Glace  from  the 
Chapeau  side  to  the  Montanvert  side.     I  then 
desired  you  to  notice  that  the  Chapeau  side 
of  the  glacier  was  more  fissured  than  either 
the  centre  or  the  Montanvert  side  (75).     Why 
should  this  be  so  ?    Knowing  as  we  now  do 
that  the  Chapeau  side  of  the  glacier  moves 
more  quickly  than  the  other,  that  the  point 
of  maximum  motion  does  not  lie  on  the  cen- 
tre but  far  east  of  it,  we  are  prepared  to  an- 
swer this  question  in  a  peifectly  satisfactory 
manner. 

279.  Let  AB  and  c  D,  in  the  following  dia- 
gram, represent  the  two  curved  sides  of  the 
Mer  de  Glace  at  the  Montanvert,  and  let  m  n 
be  a  straight  line  across  the  glacier.     Let  o 
be  the  point  of  maximum  motion.     The  me- 
chanical state  of  the  two  sides  of  the  glacier 
may  be  thus  made  plain.     Supposing  the  line 
m  n  to  be  a  straight  elastic  string  with  its 
ends  fixed  ;  let  it  be  grasped  firmly  at  tho 
point  o  by  the  finger  and  thumb,  and  drawn 
too,  keeping  the  distance  between  o'  and  the 
side  c  D  constant.     Here  the  length,  n  o  of 
the  string  would  have  stretched  to  n  o\  and 
the  length  m  o  to  m  o',  and  you  see  plainly 
that  the  stretching  of  the  short  line,  in  com- 
parison with  its  length,  is  greater  than  that 
of  the  long  line  in  comparison  with  its  length. 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


FIG.  8. 

In  other  words,  the  strain  upon  no'  is  greater 
than  that  upon  m  o' ;  so  that  if  one  of  them 
were  to  break  under  the  strain,  it  would  be 
the  short  one. 

280.  These  two  lines  represent  the  condi-, 
tious  of  strain  upon  the  two  sides  of  the  gla- 
cier.    The  sides  are  held  back,  and  the  cen- 
tre tries  to  move  on,  a  strain  being  thus  set 
up  between  the  centre  and  sides.     But  the 
displacement  of  the  point  of  maximum  mo- 
tion through  the  curvature  of    the  valley 
makes  the  strain  upon  the  eastern  ice  greater 
than  that  upon  the  western.     The  eastern 
s..".c  of  the  glacier  is  therefore  more  crevassed 
than  the  western. 

281.  Here  indeed  resides  the  difficulty  of 
getting  along  the  eastern  side  of  the  Mer  de 
Glace :  a  difficulty  which  was  one  reason  for 
our  crossing  the  glacier  opposite  to  the  Mon- 
tanvert.     There  are  two  convex  sweeps  on 
the  eastern  side  to  one  on  the  western  side, 
hence  on  the  whole  the  eastern  side  of  the 
Mer  de  Glace  is  most  riven. 

g  43.  MORAINE- RIDGES,   GLACIER  TABLES, 
AND  SAND  CONES. 

282.  When  you  and  I  first  crossed  the  "Mer 
de  Glace  from  Trelapoite  to  the  Couvercle, 
we  I'ound  that  the  stripes  of  rocks  and  rub- 
bish which  constituted  the  medial  moraines 
were  ridges  raised  above  the  general  level  of 
the  glacier  to  a  height  at  some  places  of 
twenty  or  thirty  feet.     Oil  examining  these 
ridges  we  found  the  rubbish  to  be  superficial, 
and  that  it  rested  upon  a  great  spine  of  ice 
which  ran  along  the  back  of  the  glacier.     By 
what  means  has  this  ridge  of  ice  been  raised  V 

283.  Most  boys  have  read  the  story  of  Dr. 
Franklin's  placing  bits  of  cloth  of  various 
colors  upon  snow  on  a  sunny  day.     The  bits 
of  cloth  sank  in  the  snow,  the  dark  ones 
jnost. 

284.  Consider  this  experiment.     The  sun's 
rays  first  of  all  fail  upon  the  upper  surface 
of  the  cloth  and  warm  it.     The  neat  is  then 
conducted  through  the  cloth  to  the  under 
surface,  and  the  under  surface  passes  it  oil  to 
the  snow,  which  is  finally  liquefied  by  the 
heat.     It  is  quite  manifest  that  the  quantity 
of  snow  melted  will  altogether  depend  upon 
the  amount  of  heat  sent  from  tiie  upper  to 
the  under  surface  of  the  cloth. 

285.  Now  cloth  is  what  is  called  a  bad 
conductor.     It  does  not  permit  heat  to  travel 
freely  through  it.     B  .i  where  it  has  merely 
to  pass  through  the  thickness  of  a  single  bit 
of  cloth,  a  jjood  quantity  of  the  heat  y;eta 


through.  But  if  you  double  or  treble  or 
quintuple  the  thickness  of  the  cloth  ;  or, 
what  is  easier,  if  you  put  several  pieces  one 
upon  the  other,  you  come  at  length  to  a 
point  where  no  sensible  amount  of  heat  could 
get  through  from  the  upper  to  the  under  sur- 
face. 

286.  What  must  occur   if  such  a  thick 
piece,  or  such  a  series  of  pieces  of  cloth, 
were  placed  upon  snow  on  which  a  strong 
sun  is  falling?    The  snow  round  the  cloth  is 
melted,  but  that  underneath  the  cloth  is  pro- 
tected.    If  the  action  continue  long  enough 
the  inevitable  result  will  be  that  the  level  of 
the  snow  all  round  the  cloth  will  sink,  and 
the  cloth  will  be  left  behind  perched  upon 
an  eminence  of  snow. 

287.  If  you  understand  this,  you  have  al- 
ready mastered  the  cause  of  the  moraine- 
ridges.     They  are    not    produced    by    any 
swelling  of  the  ice  upward.      But  the  ice 
underneath  the  rocks  and  rubbish  being  pro- 
tected from  the  sun,  the  glacier  right  and 
left  melts  away  and  leaves  a  ridge  behind. 

288.  Various  other  appearances  upon  the 
glacier  are  accounted  for  in  the  same  way. 
Here  upon  the  Mer  de  Glace  we  have  flat 
slabs  of  rock  sometimes  lifted  up  on  pillars 
of  ice.     These  are  the  so-called  Glacier  Ta- 
bks.     They  are  produced,  not  by  the  growth 
of  a  stalk  of  ice  out  of  the  glacier,  but  by 
the  melting  of  the  glacier  all  round  the  icu 
protected  by  the  stone.     Here  is  a  sketch  of 
one  of  the  Tables  of  the  Mer  de  Glace. 

289.  Notice  moreover  that  a  glacier  table 
is  hardly  ever  set  square  upon  its  pillar.     It 
generally  leans  to  one  side,  and  repeated  ob- 
servation teaches  you  that  it  so  leans  as  to 
enable  you  always  to  draw  the  north  and 
south  line  upon  the  glacier.     For  the  sun  be- 
ing south  of  the  zenith  at  noon  pours  its  rays 
against  the  southern  end  of  the  table,  while 
the  northern  end  remains  in  shadow.     The 
southern  end,  therefore,  being  most  warmed 
does  not  protect  the  ice  underneath  H  so  ef- 
fectually as  the  northern  end.     The  table  be- 
comes inclined,  and  ends  by  sliding  bodily 
olf  its  pedestal. 

290.  In  the  figure  opposite  we  have  what 
may  be  called  an  ideal  table.     The  oblique 
lines  represent  the  direction  of  the  sunbeams, 
and  the  consequent  tilting  of  the  table  here 
shown  resembles  that    observed    upon  t^ 
glaciers. 

291.  A    pebble    will    not    rise  thus  :  like 
Franklin's  single  bit  of  cloth,  a  dark-colored 
pebble  sinks  in  the  ice.     A  spot  of  black 
mould  will  not  rest  upon  the  surface,  but 
will  sink  ;  and  various  parts  of  the  Glacier  du 
Geant  are  honeycombed  by  the  sinking  of 
such  spots  of  dirt  into  the  ice. 

292. "But  when  the  dirt  is  of  a  thickness 
sufficient  to  protect  the  ice  the  case  is  differ- 
ent. Sand  is  often  washed  away  by  a  stream 
from  the  mountains,  or  from  the  moraines, 
and  strewn  over  certain  spaces  of  the  glacier. 
A  most  curious  action  follows  .  the  sanded 
surface  rises,  the  part  on  which  the  sand  lies 
thickest  rising  highest.  Little  peaks  and 
eminences  jut  forth,  and  when  the  distribi*- 


114 


THE  FORMS  OF  WATER 


Fro.  9. 

tkm  of  the  sand  is  favorable,  and  the  action 
wifficiently  prolonged,  you  have  little  moun- 
tains formed,  sometimes  singly,  and  some- 
times grouped  so  as  to  mimic  the  Alps 
themselves.  The  Sand  Cones  of  the  Mer  de 
Glare  are  not  striking  ;  but  on  the  Gorncr, 
the  Aletseh,  the  Morteratsch,  and  other  gla- 
cieis,  they  form  singly  and  in  groups,  reach- 
ing sonic-limes  a  height  of  ten  or  twenty 
feet. 

£  44.  THE  GLACIER  MILLS  OR  MOULINS. 


29:J  You  and  I  have  learned  by  long  ex 
poiience  the  character  of  the  Mer  de  Glace. 
•We  have  marched  over  it  daily,  with  a  defi- 
nite object  in  view,  but  we  have  not  closed 
our  eyes  to  nher  objects.  It  is  from  side 
glimpses  of  things  which  are  not  at  the  mo 
ineut  occupying  our  attention  that  fresh  sub- 
jects of  inquiry  arise  in  scientific  investiga- 
tion. 

21)4.  Thus  in  marching  over  the  ice  near 
Trclaporte  we  were  often  struck  by  a  sound 
resembling  low  rumbling  thunder.  We 
subsequently  sought  out  the  origin  of  this 
sound,  and  found  it. 

25)5.  A  large  area  of  this  portion  of  the 
glacier  is  unbroken.  Driblets  of  water  have 
room  to  form  rills,  rills  to  unite  and  form 
Rtifjims,  streams  to  combine  to  form  rush- 
iiur  brooks,  which  sometimes  cut  deep  chan- 
nels in  the  ice.  Sooner  or  later  these  streams 
reach  a  strained  portiou  of  the  glacier,  where 
a  crack  is  formed  across  the  stream.  A  way 
is  thus  opened  for  the  water  to  the  bottom  of 
the  glacier.  By  long  action  the  stream 
hollows  out  a  shaft,  the  crack  thus  becoming 
the  starting-point  of  a  funnel  of  unseen 
depth,  into  which  the  water  leaps  with  the 
sound  of  thunder. 

290.  This  funnel  and  its  cataract  form  p 
glacier  Mill  or  Moulin. 

v  2i»7.  Let  me  grasp  your  hand  firmly  while 
you  stand  upon  the  edge  of  this  shaft  and 
'look  into  it.  The  hole,  with  its  pure  blue 
shimmer,  is  beautiful,  but  it  is  terrible.  Ia- 


Fio.  10. 

cautious  persons  have  fallen  into  these 
shafts,  a  second  or  two  of  bewilderment  bo 
ing  followed  by  sudden  death.  But  caution 
upon  the  glaciers  and  mountains  ought,  by 
habit,  to  be  made  a  second  nature  to  explor- 
ers like  you  and  me. 

298.  The  crack  into  which  the  stream  first 
descended  to  form  the  moulin,  moves  down 
with  the  glacier.     A  succeeding  portion  of 
the  ice  reaches  the  place  where  the  breaking 
strain  is  exerted.      A    new   crack    is  then 
formed  above  the  moulin,  which  is  thence- 
forth forsaken  by  the   stream,  and    moves 
downward  as  an  empty  shaft.     Here  upon 
the  Mer  de  Glace,  in  advance  of  the  Grand 
Moulin  we  see  no  less  than  six  of  these  for- 
saken holes.     Some  of  them  we  sound  to  a 
depth  of  90  feet. 

299.  But  you  and  I  both  wish  to  deter- 
mine,  if  possible,  the  entire  depth  of  the  Mer 
de  Glace.    The  Grand  Moulin  offers  a  chance 
of  doing  this  which  we  must  not  neglect 


IN  CLOUDS  AND  RIVERS.  ICE  AND  GLACIERS. 


Our  tlrst  effort  to  sound  the  moulin  fails 
through  the  breaking  of  our  cord  by  the  im- 
petuous plunge  of  tho  water.  A  lump  of 
grease  in  the  hollow  of  a  weight  enables  a 
mariner  to  judge  of  a  sea  bottom.  We  em- 

C"    '  such  a  weight,   but  cannot  reach  the 
of  the  g'uc  er.     A  depth  of  163  feet  is 
the  utmost  rtajhed  by  our  plummet. 

300.  From  July  28th  to  August  8th  we  have 
watched  the  progress  of  the  Grand  Moulin. 
On  the  former  date  the  position  of  the  mou- 
lin  was  fixed.  On  the  31st  it  had  moved 
down  50  inches  ;  a  little  more  than,  a  day 
afterward  it  had  moved  74  inches.  On 
August  8lh  it  had  moved  198  inches,  which 
gives  an  average  of  about  18  inches  in 
twenty  four  hours.  No  doubt  next  summer 
upon  the  Mer  de  Glace  a  Grand  Moulin  will 
be  found  thundering  near  Trolaporte  ;  but 
like  the  crevasse  of  Hie  Grand  Plateau,  al- 
ready referred  to  (£  1(5),  it  will  not  be  our 
moulin.  This,  or  i  at  her  the  ice  which  it 
penetrated,  is  now  probably  more  than  a 
mile  lower  down  than  it  was  in  1857. 

§  45.  THE  CHANGES  OF  VOLUME  OF  WATER 

BY  HEAT  AND  COLD. 

SOI.  We  have  noticed  upon  the  glacier 
shafts  and  pits  filled  with  water  of  the  most 
delicate  blue.  In  some  cases  these  have  been 
the  shafts  of  extinct  moulins  closed  at  the 
bottom.  A.  theory  has  been  advanced  to  ac- 
count for  them,  which,  though  it  may  be  un- 
tenable, opens  out  considerations  regarding 
'.he  properties  of  water  that  ought  to  be 
familiar  to  inquirers  like  you  and  me. 

302.  In  our  dissection   of  lake  ice  by   a 
beam  of  heat  (£  11)  we  noticed  little  vacu- 
ous spots  at  the  centres  of  the  liquid  flowers 
formed  by  the  beam.     These  spots  we  re- 
ferred to  the  fact  that  when  ice  is  melted  the 
water  produced  is  less  in  volume  than  the 
ice,  and  that  hence  the  water  of  the  flower 
was  not  able  to  occupy   the    whole    space 
covered  by  the  flower. 

303.  Let  us  more  fully  illustrate  this  sub- 
ject.    Stop  a  small  flask  water-tight  with  a 
cork,    and    through    the    cork    introduce  a 
narrow  glass  tube  also  water-tight.       It  is 
easy  to  fill  the  flask  with  water  "so  that  the 
liquid  shall  stand  at  a  certain  height  in  the 
glass  tube. 

304.  Let  us  now  warm  the  flask  with  the 
flame  of  a  spirit-lamp.     On  first  applying  the 
flame  you  notice  a  momentary  sinking  of  the 
liquid  in  the  glass  tube.     This  is  due  to  the 
momentary  expansion  of  the  flask  by  ha.t  ; 
it  becomes  suddenly  larger  when  the  flame  is 
first  applied. 

305.  But  the  expansion  of  the  water  soon 
overtakes  that  of  the  flask  and  surpasses  it. 
We  immediately  see  the  rise  of  the  liquid 
column  in  the  glass  tube,  exactly  as  mercury 
rises  in  the  tube  of  a  watmed  thermometer. 

800.  Our  glass  tube  is  ten  inches  long,  and 
at  starting  the  water  stood  in  it  at  a  height  of 
five  inches.  We  will  apply  the  spirit-lamp 
flame  until  the  water  rises  quite  to  the  top  of 
the  tube  and  trickles  over.  This  experiment 
suffices  to  show  the  expansion  of  the  wale? 


by  heat. 

307.  We  now  take  a  common  finger-glass 
and  put  into  it  a  little  pounded  ice  and  "salt. 
On  this  we  place  the  flask,  and  ihen  build 
round  it  the  freezing  mixture.  The  liquid 
column  retreats  dosvn,  the  tube,  proving  the 
contraction  of  the  liquid  by  cold.  We  allow 
the  shrinking  to  continue  for  some  minutes, 
noticing  that  the  downward  retreat  of  the 
liquid  becomes  gradually  slower,  and  that  it 
finally  ceases  altogether. 

308~.  Keep  your  eye  upon  the  liquid  col- 
umn ;  it  remains  quiescent  for  a  fi action  of 
a  minute,  and  then  moves  once  more.  But 
its  motion  is  now  upward  instead  of  down- 
ward. The  freezing  mixture  now  acts  exactly 
li/ce  theflatne. 

309.  It  would  not  be  difficult  to  pass  a 
thermometer     through    the    cork    inio    the 
flask,  and  it  would  tell  us  the  exact  tempera- 
ture at  which  the  liquid  ceased  to  coii tract 
and  began  to  expand.     At  that  mumtnt  we 
should  find  the  temperatuic  of  the  liquid  a 
shade  over  39°  Fahr. 

310.  At  this  temperature,  then,  water  at- 
tains its  maximum  density. 

311.  Seven  degrees  below  this  temperature, 
or  at  32°  Fahr.,  the  liquid  begins  to  turn  into 
solid  crystals  of  ice,  which  you  know  swims 
upon  water  because  it  is  bulkier  for  a  given 
weight.     In  fact,  this  halt  of  the  apprracb- 
ing  molecules  at  the  temperature  of  39",  is 
but  the  preparation  for  the  subsequent  act  of 
crystallization,  in  which    the  expansion  by 
cold  culminates.     Up  to  the  point  of  solidin> 
cation  the  increase  of  volume  is  slow  and 
gradual  ;  while  in  the  act  of  solidification  it 
is  sudden,  and  of  overwhelming  strength. 

312.  By  this  force  of  expansion  the  Floren- 
tine Academicians  long  ago  burst  a  sphere 
of  copper  nearly  three  quaiters  of  an  inch  in 
thickness.     By  the  same  force  the  celebrated 
astronomer  Huyghens  burst  in  1GG7  iron  can- 
nons a  finger  breadth  thick.     Such  experi- 
ments have    been    frequently    made    since. 
Major  Williams  during  a  severe  Quebec  win- 
ter filled  a  mortar  with  water,  and  closed  it 

i  by  driving  into  its  muzzle  a  plug  of  wood. 

i  Exposed  to  a  temperature  50  Fahr.  below 
the  freezing  point  of  water,  the  mKal  resist- 
ed the  strain,  but  the  plug  gave  wy.y,  b(  ing 
projected  to  a  distance  of  400  feet.  At 
Warsaw  howitzer  shells  bave  been  thus  ex- 
ploded ;  and  you  and  I  have  shivered  thick 
bomb-shells  to  fragments  by  placing  them 
for  half  an  hour  in  a  freezing  m'xture. 

313.  The  theory  of  the  shafts  and  pits  re- 
ferred to  at  the  beginning  of  inis  section  is 
this  :  The  water  at  the  surface  of  the  shaft 
is  warmed  by  the  sun,  say  to  a  temperature 
of  39°  Fahr.     The  water  at  the  I  ottom,  in 
contact  with  the  ice,  must  be  at  32°  or  near 
it.     The  heavier  water  is  therefore  at  the 
top  ;  it  will  descend  to  the  bottom,  melt  the 
ice  there,  and  thus  deepen  the  shaft. 

314.  The  circulation  here  refeired  to  un- 
doubtedly goes  on,  and  some  curious  ejects 
are  due  to  it ;  but  not,  I  think,  the  one  here 
ascribed  to  it.     The  deepening  of  a  shaft  im- 
plies a  quicker  melting  of  its  bottom  than  of 


115 


THE  FORMS  OF  WATER 


the  surface  of  the  glacier.  It  is  not  easy  to 
see  how  the  fact  of  the  solar  heat  being  first 
absorbed  by  water,  and  then  conveyed  by  it 
10  the  bottom  of  the  shaft,  should  make  the 
melting  of  the  bottom  more  rapid  than  that 
of  the  ice  which  receives  the  direct  impact 
of  the  solar  rays.  The  surface  of  the  glacier 
must  sink  at  least  as  rapidly  as  the  bottom  of 
the  pit,  so  that  the  circulation,  though  actu- 
ally existing,  cannot  produce  the  cited  as- 
cribed to  it. 

§  40.  CONSEQUENCES  FLOWING  FROM  THE 
FOREGOING    PROPERTIES    OF    WATER. — 
CORRECTION  OF  ERRORS. 

315.  I  was  not  much  above  your  age  when 
the  pioperly  of  water  ceasing  to  contract  by 
cold  at  a  temperature  of  39"  Fahr.  was  made 
known  to  me,  and  1  still  remember  the  im- 
pression it  made  upon  me.     For  I  was  asked 
to  consider  what  would  occur  in  case  this 
solitary  exception  to  an  otherwise  universal 
law  ceased  to  exist. 

316.  I  was  asked  to  reflect  upon  the  con- 
dition of  a  lake  stored  with  fish  and  offering 
its  surface  to  very  cold  air.     It  was  made 
clear  to  me  that  the  water  on    being  first 
chilled  would  shrink  in  volume  and   become 
heavie/,  that  it  would    theiefore    sink    and 
have  its;  p'ate  supplied  by  the  warmer  and 
lighter  wati-r  fiom  the  deeper  portions  of  the 
lake. 

317.  It  was  pointed  out  to  mi;  that  without 
the  law  referred  to  this  process  of  circulation 
would  go  on  until   th^  whole  watir  of  the 
lake  had  been  lowered  to  the  free/ing  tem- 
per ami  e.       Congelation  would  then  begin, 
and  would  continue  as  long  as  any  waler  re- 
mained to  be  solidified.     One  consequence  of 
this  would  be  to  destroy  every  living  thing 
contained  in    the    lake.       Other    calamities 
weie  added,  all  of  which  weie  said  to  be 
prevented  by  the  perfectly  exceptional  ar- 
rangement, that  after  a  certain  time  the  colder 
water  becomes  the  lighter,  floats  on  the  sur- 
face of  the  lake,  is  there    congealed,   thus 
throwing  a    protecting    roof    over    the  life 
below. 

318.  Count  Rumford,  one  of  the  most  solid 
of  scientific  men, writes  in  the  following  strain 
about  this  question  :    ''  It  does  not  appear  to 
me  that  there  is    anything    which     human 
sagacity  can  fathom,  within  the  wide-extend- 
ed bounds  of    the    visible    creation,    which 
affoidsa  more   striking    or   more  palpable 
proof  of  the  wisdom  of  the  Creator,  and  of 
the  special  care  He  has  taken,  in  the  general 
arrangement  of  the  uuiveise,  to  preserve  ani- 
mal M'e,  than  this  wonderful  contrivance. 

31S).  "  Let  me  beg  the  attention  of  my 
leaders  while  1  endeavor  to  investigate  this 
imobt  interesting  subject ;  and  let  me  at  the 
:*umc  time  bespeak  his  candor  and  indul- 
gence. I  feel  the  danger  to  which  a  mortal 
•exptMses  himself  who  has  the  temerity  to  ex- 
plain the  designs  of  Infinite  Wisdom.  The 
enterprise  is  adventuious,  but  it  surely  can- 
not be  improper. 

320.  "Had  not  Providence  interfered  on 
iiua  occasion  in  a  manner  which  may  well 


be  considered  as  m^'acul<mst  all  the  fresh 
water  within  the  polar  circle  must,  inevitably 
have  been  frozen  to  a  veiy  great  depth  iu 
winter,  and  every  plant  ami  tree  destroyed." 

321.  Through  many  pages  of    his    book 
Count  Rumford  continues  in  \\\\»  strain  to 
expound  the  ways  and  intentions  of  the  Al- 
mighty, and  he  does  cot  hesitate  to  apply 
very  harsh  words  to  those  who  cannot  share 
his  notions.    He  calls  them  hardened  and  de- 
graded.    \Ve  aie  here  .warned   :>f  the  fact, 
which  is  too  often  forgotten,  that  tlie  pleas- 
ure or  comfort  of  a  belief,  or  the  warmth  or 
exaltation  of  feeling  which  it  produces,  is  no 
guarantee  of  its  truth.     For  the  whole  of 
Count  Rumford's  delight  and  enthusiasm  in 
connection  with  this  subjact,  and  the  wholo 
of  his  ire  against  those  who  did  not  share  his 
opinions,  were  founded  upon  an  erroneous 
notion. 

322.  Water  is  not  a  solitary  exception  to 
an  otherwise  general  law.     There  are  other 
molecules  than  those  of  this  lieniid  which  re- 
quire more  room  in  the  soliel  c-ijstalline  con- 
dition than  in  the  adjacent  molten  condition. 
Iron  is  a  case  in  point.       Solid  iron  floats 
upon  molten  iron  exactly  as  ice  floats  upon 
water.     Bismuth  is  a  still  more  impressive 
case,  and  we  could  shiver  a  bomb  as  cer- 
tainly by  the  solidification  of  bismuth  as  by 
that  of  water.     There  is  no  fish  to  be  taken 
care  of  here,  still  the  "  contrivance"  is  the 
same. 

323.  I  am    reluctant  to  mention  (hem  iu 
the  same  biealh  with  Count  liumioid,  but  I 
am  told  that  in  our  own  day  theie  are  people 
who  profess  to  find  the  comforts  of  a  religion 
in  a  superstition  lower  than  any  that  has 
hitherto  degraded  the  civilized  human  mind. 
So  that  the  happiness  of  a  faith  and  the  truth 
of  a  failh  are  two  totally  different  things. 

324.  Life  and  the  conditions  of  life  are  in 
necessary  haimony.     This  is  a  truism,  for 
without  the  suitable  conditions  life  could  not 
exist.     But  both  life  and  its  conditions  set 
forth  the  operations  of  inscrutable  Power. 
We  know  not  its  origin  ;  we  know  not  its 
end.     And  the  presumption,  if  not  the  eleg- 
radation,  rests  with  those  who  place  upon 
the  throne  of  the  universe  a  magnified  image 
of  themselves,  and  make  its  doings  a  mere 
colossal  imitation  of  their  own. 

§  47.  THE  MOLECULAR  MKC/TANISM  OF 
WATER-CONGELATION. 

82.1.  But  let  us  return  to  our  science. 
How  are  we  to  picture  this  act  of  expansion 
on  the  part  of  freezing  water  ?  By  what  oper- 
ation do  the  molecules  demand  wilh  such 
irresistible  emphasis  more  room  in  the  solirl 
than  in  the  adjacent  liquid  condition  ?  In  all 
cases  of  this  kiud  we  must  derive  our  con. 
ceptions  from  the  world  of  the.  senses,  and 
transfer  them  afterward  to  a  world  transcend- 
ing the  range  of  the  senses. 

32(>.  You  have  not  forgotten  our  conver- 
sation regarding  "  atomic  poles"  (^  10),  and 
how  the  notion  of  polar  force  came  to  be  ap- 
plied to  crystals.  With  this  fiesh  in^  your 
memory,  you  will  have  no  great 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


117 


Ft? 


In  understanding  how  expansion  of  volume 
may  accompany  tho  act  of  crystallization. 

327.  I  place  a  number  of  magnets  before 
,  ju.     Thpy,  as  matter,  are  affected  by  grav- 
ty,  and,  if  perfectly  free,  they  would  move 

toward  each  other  in  obedience  to  the  attrac- 
tion of  gravity. 

328.  But  they  are   not  only  matter,  but 
magnetic  matter.      They  not  only  act  upon 
each  other  by  the  simple  force  of  gravity, 
but  by  the  polar  force  of  magnetism.     Im- 
agine them  placed  at  a  distance  from  each 
other,  and  perfectly  free  to  move.     Gravity 
first  makes  itself  felt  and  draws  them  to- 
gether.    For  a  time  the  magnetic  force  issu- 
ing from  the  poles  is  insensible  ;  but  when  a 
certain  nearness  is  attained,  1he  polar  force 
comes  into  play.     The  mutually  attracting 
points  close  up,  the  mutually  repellent  points 
retreat,  and  it  is  easy  to  see  that  this  action 
may  produce  an  arrangement  of  the  magnets 
which  requires  more  room.     Suppose  them 
surrounded  by  a  box  which  exactly  incloses 
them  at  the  moment  the    polar  force  first 
comes  into  play.     It  is  easy  to  see  that  in  ar- 
ranging themselves  subsequently  the  repelled 
corners  and  ends  of  the    magnets  may  be 
caused  to  nress  against  the  Bides  of  the  box, 
and  even  to  burst  ii,  if  the  forces  be  suffi- 
ciently strong. 

329.  Here  then  we  have  a  conception  which 
may  be  applied  to  the  molecules  of  water. 
They,  like  the  magnets,  are  acted  upon  by 
two  distinct  forces.     For  a  time,  while  thr». 


FIG.  11. 

liquid  ia  being  coole.l  they  approach  each 
other,  in  obedience  to  their  general  uttractioo 
for  each  other.  But  at  a  certa!  i  point  new 
"**ces,  some  attractive,  some  repulsive,  ema^ 


noting  from  special  points  of  the  molecules, 
come  into  plav.  The  attracted  points  closa 
up,  the  repelled  points  retreat.  Thus  tLo 
molecules  turn  and  rearrange  themselves, 
demanding,  as  they  do  so,  more  space,  and 
overcoming  all  ordinary  resistance  by  the 
energy  of  their  demand.  This,  in  general 
terms,  is  an  explanation  of  the  expansion  of 
water  in  solidifying  :  it  would  be  easy  to 
construct  an  apparatus  for  its  illustration. 

§  48.  THE  DIKT  BANDS  OF  THE  MEII  DE 
GLACE. 

330.  Pass  from  bright    sunshine    into  a 
moderately  lighted  room  ;  for  a  time  all  ap- 
pears so  dark  that  the  objects  in  the  room  are 
not  to  be  clearly  distinguished,     Hit  violent- 
ly by  the  waves  of  light  (§  3)  the  optic  nervo 
is  numbed,  and  requires  time  to  recover  its 
sensitiveness. 

331.  It  is  for  this  reason  that  I  choose  the 
present  hour  for  a  special  observation  on  iho 
Mer  de  Glace.     The  sun  has  sunk  behind  the 
ridge  cf  Charmoz,  and  the  surface  of  the 
glacier  is  in  sober  shade.     The  main  portion 
of  our  day's  work  is  finished,  but  we  have 
still  sufficient  energy  to  climb  tho  slopes  ad- 
jacent to  the  Montanvert  to  a  height  of  a 
thousand  feet  or  thereabout  above  the  ice. 

332.  We  now  look  fairly  down  upon  the 
glacier,  and  see  it  less   foreshortened  than 
from  the   Montanvert.     We  notice  the  dirt 
overspreading    its  eastern  side,  due  to  the 
crowding  together  of  its  medial   moraines. 
We  see  the  comparatively  clean  surface  of 
the  Glacier  du  Geant ;  but  we  notice  upon 
tills  surface  an  appearance  which  we  have 
not  hitherto  seen.     It  is  crossed  by  a  series 
of  gray  bent  bands,  which  follow  each  other 
in  "succession,  from  Trelaporte  downward. 
We  count  eighteen  of  theso  from  our  present 
position.     (See  sketch.  Fig.  12.) 

333.  These  are  the  Dirt  Bands  of  the  Mcr 
<le  Glace  ;  they  were  first  observed   by  Pro- 
fessor Forbes  in  18i2. 

334.  They  extend  down  theglackr  further 
than  we  can  sec  :  and  if  we  cross  the  valley 
of    Chamouni,    r,ml    climb    the    mountains 
;U  the   opposite   side,  to  a  point    near  the 
little  auberge,  called  La  Fle'gere,  we  shall 
command  a  view  of  the  end  cf  the  glacier 
and  observe  the  completion  of  the  series  cf 
bands.      We  notice  that  they  are  confined 
throughout  to  the  portion  of  the  glacier  do- 
rived  from  the  Col  du  Geant.     (See  sketch, 
Fig.  11.) 

335.  We  must  trace  them  to  their  source. 
You  know  how  noble  and  complete  a  view 
is  obtained  of  the  glacier  and  Col  de  Gt'*iut 
from    the    Cleft   Station  above    Trelaporte. 
Thither    we    must    once  more  climb  :  <uul 
thence  we  can  see  the  succession  of  Lands 
stretching    downward    to    the    Montanvert, 
and  upward  to  the  base  of  tho  ice-casciuile 
upon  the  Glacier  du  Ge'ant.     The  cascade  is 
evidently  concerned  in  their  formation.    (See 
Bketch,  Fig.  13.) 

336.  And  how  ?   Simply  enough.    The  gla- 
cier, as  we  know,  is  broken  transversely  a,t  the 
summit  of  the  ice- fall,  and  descends  the  do 


1115 


THE  FOKMS  OF  WATEIi 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


119 


clivity  in  a  series  of  great  transverse  ridges. 
At  the  base  of  the  fall,  the  chasms  are 
closed,  but  the  ridges  in  part  remain,  forming 
protuberances,  which  run  like  vast  wrinkles 
across  the  glacier.  These  protuberances  are 
more  and  more  bent  because  of  the  quicker 
motion  of  the  centre,  and  the  depressions  be- 
tween them  form  receptacles  for  the  fine 
mud  and  ddbris  washed  by  the  little  rills  from 
tLe  adjacent  slopes. 

'•337.  The  protuberances  sink  gradually 
through  the  wasting  action  of  the  sun.  so 
that  long  before  Tre'laporte  is  reached  they 
have  wholly  disappeared.  Not  so  the  dirt  of 
which  they  were  the  collectors  :  it  continues 
to  occupy,  in  transverse  bands,  the  flat  sur- 
face of  the  glacier.  At  Trelaporte,  moreover, 
where  the  valley  becomes  narrow,  the  bands 
are  much  sharpened,  obtaining  there  the 
character  which  they  afterwaul  preserve 
throughout  the  Mer  de  Glace.  Other  glaciers 
with  cascades  also  exhibit  similar  bands. 

§  49.  SEA  ICE  AND  ICEBERGS. 

338.  We  are  now  equipped  intellectually 
for  a  campaign  into  another  territory.     Water 
becomes  heavier  and  more  difficult  to  freeze 
when  salt  is  dissolved  in  it.     Sea  water  is 
therefore  heavier  than  fresh,  and  the  Green- 
land Ocean  requires  to  freeze  it  a  temperature 
3£  degrees  lower  than  frish  water.     When 
concentrated  till  its  specific  gravity  reaches 
1.1045,  sea  water  requires  for  its  congelation 
a  temperature  1S£  degrees  lower  than  the  or- 
dinary freezing-point. 

339.  But  even  when  the  water  is  saturated 
with  salt,  the  crystallizing  force  studiously 
rejects  the  salt,  and  devotes  itself  to  the  con- 
gelation of  the  water  alone.     Hence  the  ice 
of  sea  water,  when  melted,  produces  fresh 
water.     The  only  saline  particles  existing  in 
such  ice  are  those  entangled  mechanically  in 
its  pores.     They  have  no  part  or  lot  in  the 
structure  of  the  crystal. 

340.  This  exclusiveness,  if  I  may  uso  the 
term,  of  the  water  molecules  ;  this  entire  re- 
jection of  all  foreign  elements  from  the  edi- 
fices which  they  build,  is  enforced  to  a  sur- 
prising degree.     Sulphuric  acid  has  so  strong 
an  affinity  for  water  that  it  is  one  of  the  most 
powerful  agents  known  to  the  chemist  for 
the  removal  of  humidity  from  air.     Still,  as 
shown  by  Faraday,  when  a  mixture  of  sul- 
phuric  acid  and  water  is  frozen,  the  crys- 
tal formed  is  perfectly  sweet  and  free  from 
acidity.     The  water  alone  has  lent  itself  to 
tho  crystallizing  force. 

341.  Every  winter  in  the  Arctic  regions 
the  sea  freezes,  roofing  itself  with  ice   of 
enormous  thickness  and  vast  extent.     By  the 
summer  heat,  and  the  tossing  of  the  waves, 
this  is  broken  up  ;  the  fragments  are  drifted 
by    winds    and  borne  by  currents.      They 
clash,  they  crush  each  other,  they  pile  them- 
selves into  heaps,  thus  constituting  the  chief 
danger  encountered  by  mariners  in  the  polar 
seas. 

342.  But  among  the  drifting  masses  of  flat 
sea-ice,  vaster  masses  sail,  which  spring  from 
*  totally  different  source      These  are  the  lc&- 


bergs  of  the  Arctic  seas.  They  rise  some- 
times  to  an  elevation  of  hundreds  of  fet't 
above  the  water,  while  the  weight  of  ice  sub- 
merged is  about  seven  times  that  seen  above. 

343.  The  first  observers  of  striking  natural 
phenomena  generally  allow  wonder  and  im- 
agination more  than  their  due  place.     But  to 
exclude  all  error  arising  from  this  cause,  I 
will  refer  to  the  journal  of  a,  cool  and  intrepid 
Arctic  navigator,  Sir   Leopold  McClintock. 
He  describes  an  iceberg  250  feet  high,  which 
was  aground  in  500  feet    of   water.      This 
WoUld  make  the  entire  height  of  the  berg  75$ 
feet,  not  an  unusual  altitude  for  the  greater 
icebergs. 

344.  From  Baffin's  Bay  1  hese  mighty  masses 
come  sailing  down  through  Davis'  Straits  into 
the  broad  Atlantic.     A  vast  amount  of  heat 
is  demanded  for  the  simple  liquefaction  of  ice 
(£  48)  ;  and  the  melting  of  icebergs  is  on  this 
account  so  slow,  that  when  large  they  some- 
times maintain  themselves  till  they  have  been 
drifted  2000  miles  from  their  place  of  birth. 

345.  What  is  their  origin?     The  Arctic 
glaciers.     From  the  mountains  in  the  interior 
the  indurated  snows  slide  into  the  valleys  and 
fill  them  with  ice.     The  glaciers  thus  formed 
move  like  the  Swiss  ones,  incessantly  down- 
ward.    But  the  Arctic  glaciers  reach  the  sea, 
enter  it,  often  ploughing  up  its  bottom  into 
submarine  moraines.      Undermined  by  the 
lapping  of  the  waves,  and  unable  to  resist  the 
strain  imposed  by  their  own  weight,  they 
break  across,  and  discharge  vast  masses  into 
the  ocean.     Some  of  these  run  aground  on 
the  adjacent    shores,    and    often    maintain 
themselves  for  years.     Others  escape  south- 
ward,  to  be  finally  dissolved  in  the  warm 
waters  of  the  Atlantic.     The  first  engraving 
on  this  pasje  is  copied   from  a  photograph 
taken  by  Mr.  Bradford  during  a  recent  ex- 
pedition to  the  Northern  seas.     The  second 
represents  a  mass  of  ice  upon  the  Glacier 
des  Bossons.     Their  likeness  suggests  their 
common  origin. 

§  50.  THE  ^GGISCHHORN,  THE  MABGELIN 
AND  ITS  ICEBEHGS. 


346.  I  am,  however,  unwilling  that  you 
should  quit  Switzerland  without  seeing  such 
icebergs  as  it  can  show,  ?nd  indeed  there  are 
other  still  nobler  glaciers  than  the  Mer  de 
Glace    with    which    you    ought  to  be  ac- 
quainted.      In    tracing    the    Rhone    to    its 
source,  you  have  already  ascended  the  valley 
of  the  Rhone.     Let  us  visit  it  again  together  ; 
halt  at  the  little  town  of  Viesch,  and  go  from 
it  straight  up  to  the  excellent  hostelry  on  the 
slope  of  the  ^Eggischhorn.      This  we  shall 
make    our  headquarters  while   we   explore 
that  monarch  of  European  ice-streams  —  the 
great  Aletsch  glacier. 

347.  Including  the  longest  of  its  branches, 
this  noble  ice-river  is  about  twenty  miles 
long,  while  at  the  middle  of  its  trunk  it  meas- 
ures nearly  a  mile  and  a  quarter  from  side 
to  side.     The  grandest  mountains  of  the  Ber- 
nese Oberland,  the  Jun.irf'rau,  the  Monch,  the 
Trusberg,  the  Aletschhorn,  the  Breithorn, 
the  Gletscherhorn,  and  many  another  noble 


130 


THE  FORMS  OF  WATER 


peak  and  ridge,  arc  the  collectors  of  its  neves. 
From  three  great  valleys  formed  in  the  heart 
of  the  mountains  these  neves  are  poured, 
uniting  together  to  form  the  trunk  of  the 
Aletsch  at  a  place  named  by  a  witty  moun- 
taineer, the  "  Place  de  la  Concorde  of  Na- 
ture." If  the  phrase  be  meant  to  convey 
the  ideas  of  tranquil  grandeur,  beauty  of 
form,  and  purity  of  hue,  it  is  well  bestowed. 
JB4.S.  Our  hotel  is  not  upon  the  peak  of  the 
,/Eiggischhorn,  but  a  brisk  morning  walk 
soon  places  us  upon  the  top.  Thence  we  see 
the  glacier  like  a  broad  river  stretching  up- 
ward to  the  roots  of  the  Jungfrau,  and 
downward  past  the  Bel  Alp  toward  its  end. 
Prolonging  the  vision  downward,  we  strike 
the  noblest  mountain  group  in  all  the  Alps — 
the  Dora  and  its  attendant  peaks,  the  Matter- 


horn  and  the  Weisshorn.  The  scene  indeed 
is  one  of  impressive  grandeur,  a  multitude  of 
peaks  and  crests  here  unearned  contributing 
to  its  glory. 

840.  But  low  down  to  our  right,  and  sur- 
rounded by  the  sheltering  mountains,  is  an 
object  the  beauty  of  which  startles  those  who 
are  unprepared  for  it.  Yonder  we  see  the 
naked  side  of  the  glacier,  exposing  glistening 
ice-cliffs  sixty  or  seventy  feet  high.  It 
would  seem  as  if  the  Aletsch  here  were  en- 
gaged in  the  vain  attempt  to  thrust  an  arm 
through  a  lateral  valley.  It  once  did  so  ;  but 
the  arm  is  now  incessantly  broken  off  close 
to  the  body  of  the  glacier,  a  great  space  for- 
merly covered  by  the  ice  being  occupied  by 
its  water  of  liquefaction.  A  lake  of  the  love- 
liest blue  is  thus  formed,  which  reaca^  quit® 


Fio.  14. 


Fro. 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


13Z 


to  the  base  of  the  ice-cliffs,  saps  them,  as  the 
Arctic  waves  sap  the  Greenland  glaciers,  and 
receives  from  them  the  broken  masses  which 
it  has  undermined.  As  we  look  down  upon 
the  lake,  small  icebergs  sail  over  the  tranquil 
surface,  each  resembling  a  snowy  swan  ac- 
companied by  its  shadow. 

350.  This  is  the  beautiful  little  lake  of 
Margeliu,  or,  as  the  Swiss  here  call  it,  the 
Margolin  See.  You  see  that  splash,  and  imme- 
diately afterward  hear  the  sound  of  the  plung- 
ing ice.  Tho  glacier  has  broken  before  our 
eyes,  and  dropped  an  iceberg  into  the  lake. 
All  over  the  lake  the  water  is  set  in  commo- 
tion, thus  illustrating  on  a  small  scale  the 
swamping  waves  produced  by  the  descent  of 
vast  islands  of  ice  from  the  Arctic  glaciers. 
Look  to  the  end  of  the  lake.  It  is  cumbered 
with  the  remnants  of  icebergs  now  agiouad, 
which  have  been  in  part  watted  thither  by 
the  wind,  but  in  part  slowly  borne  by  the 
water  which  moves  gently  in  this  direction. 

851.  Imagine  us  below  upon  the  margin  of 
the  lake,  as  I  happened  to  be  on  one  occasion. 
There  is  one  large  and  lonely  iceberg  about 
the  middle.  Suddenly  a  sound  like  that  of  a 
cataract  is  heard  ;  we  look  toward  tha  ice- 
berg and  see  water  teeming  from  its  sides. 
Whence  comes  the  water  ?  the  berg  has  be- 
come top-heavy  through  the  melting  under- 
neath  ;  it  is  in  the  act  of  performing  a  somer- 
sault, and  in  rolling  over  carries  with  it  a 
vast  quantity  of  water,  which  rushes  like  a 
waterfall  down  its  sides.  And  notice  that  the 
iceberg,  which  a  moment  ago  was  snowy- 
white,"  now  exhibits  the  delicate  blue  color 
characteristic  of  compact  ice.  It  will  soon, 
however,  be  rendered  white  again  by  the  ac- 
tion of  the  sun.  The  vaster  icebergs  of  the 
Northern  seas  sometimes  roll  over  in  the 
same  fashion.  A  week  may  be  spent  with 
delight  and  profit  at  the  jEggischhorn. 

§  51.  THE  BEL  ALP. 

35.2.  From  the  ,Eggisehhorn  I  might  lead 
you  along  tlu:  mountain  ridge  by  the  Bel.ten 
See,  tha  fish  of  which  we  have  already 
tasted,  to  the  Rieder  Alp,  and  thence  across 
the  Aletsch  to  the  Bel  Alp.  This  is  a  fine 
mountain  ramble,  but  you  and  I  prefer  mak- 
ing the  glacier  our  'highway  downward. 
Easy  at  some  places,  it  is  by  no  means  child's 
play  at  others  to  unravel  its  crevasses.  But 
the  steady  constancy  and  close  observation 
which  we  have  bitherto  found  availing  in 
difficult  places  do  not  forsake  us  here.  We 
clear  the  fissures  ;  and,  after  four  hours  of 
exhilarating  work,  we  find  ourselves  upon  the 
slope  lending  up  to  the  Bel  Alp  hotel. 

853.  This  is  one  of  the  finest  halting-places 
in  the  Alps.  Stretching  before  us  up  to  the 
^Eggischhorn  and  Margelin  See  is  the  long 
last  reach  of  the  Aletsch,  with  its  great  me- 
dial moraine  running  along  its  back.  At  hani 
is  the  wild  gorge  of  the  M-issa,  in  which  the 
snout  of  the  glacier  lies  couched  like  the  head 
of  a  serpent.  The  beautiful  system  of  tha 
Oberaletsch  glaciers  is  within  easy  reach. 
Above  us  is  a  peak  called  the  Sparrenhorn, 
"•ccessible  t.o  the  most  moderate  climber,  and 


on  the  summit  of  which  little  more  than  an 
hour's  exertion  will  place  you  and  me.  Below 
us  now  is  the  Oberaletsch  glacier,  exhibiting 
the  most  perfect  of  medial  moraines.  Near  us 
is  the  great  mass  of  the  Aletsch  horn,  clasped 
hy  its  neve's,  and  culminating  in  brown  rock. 
It  is  supported  by  other  peaks  almost  as  noble 
as  itself.  The  Nestliorn  is  at  hand  ;  while 
sweeping  round  to  the  west  we  strike  the 
glorious  triad  already  referred  to,  the  Weiss- 
horn,  the  Maiterhorn,  and  the  Dom.  Take 
one  glance  at  the  crevasses  of  the  glacier  im- 
mediately below  us.  It  tumbles  at  its  end 
down  a  Bteep  incline,  and  is  greatly  riven. 
But  the  crevasses  open  before  the  steep  part 
is  reached,  ami  you  notice  the  coalescence  of 
marginal  and  transverse  crevasses,  produc- 
ing a  system  of  curved  fissures  with  the  con- 
vexities of  the  curves  pointing  upward. 
The  mechanical  reason  of  this  is  now  known 
to  you.  The  glacier-tables  are  also  numerous 
and  line.  I  should  like  to  linger  with  you 
here  for  a  week,  exploring  the  existing  gfa- 
ciers.  and  tracing  out  the  evidences  of  others 
that  have  passed  away. 

§  52.    THE  RIFFELBEKG  AND  GORNER 
GLACIER. 

354.  And  though  our  measurements  and 
observations  on  the  Mer  de  Glace  are  more 
or  less  representative  of  all  that  can  be  made 
or  solved  elsewhere,  I  am  unwilling  to  leave 
you  unacquainted  with  the  great  system  of 
glaciers    which    stream    from  the   northern 
slopes  of  Monte  R^sa  and  the  adjacent  moun- 
tains.    From  the  Bel  Alp  we  can  descend  to 
Brieg,  and  thence  drive  to  Visp  ;  but  you 
anl  Lprefer  the  breezy  heights,  so  we  sweep 
round 'the  promontory  of  the  Nessel,  until  we 
stand  over  the  Rhone  valle3r,  in  front  of  Visp. 
From  this  village  an  hour's  walking  carries 
us  to  Stalden,  where  the  valley  divides  into 
two  branches  :  the  one  leading  through  Saaa 
over  the  Monte  Moro,  and  the  other  througij 
St.  Nicholas  to  Zeimatt.     The  latter  is  our 
route. 

355.  We  reach  Zermatt,  but  do  not  halt 
there.      On  the  mountain  ridge,  4000  feet 
above  the  valley,  we  discern  the  Riffelberg 
hotel.     This  we  reach.     Right  in  front  of  us 
is  the  pinnacle  of  the  Matterhorn,  upon  the 
top  of  which  it  must  appear  incredible  to  you 
that  a  human  foot  could  ever  tread.     Con- 
stancy and  skill,  however,  accomplished  this, 
but  in  the  first  instance  at  a  terrible  price.    In 
the  little  churchyard  of  Zermatt  we  have  seen 
the  graves  of  two  of  the  greatest  mountaineers 
that  Savoy  and  England  have  produced  ;  and 
who,  with  two  gallant  young  companions,  fell 
from  the  Matterhorn  in  1865. 

356.  At  the  Riffelberg  we  are  within  an 
hour's  walk  of  the   famous    Gorner    Grat, 
which  commands  so  grand   a  view  of  the 
glaciers  of  Monte  Rosa.     But  yonder  huge 
knob  of  perfectly  bare  rock,  which  is  called 
the  Riffelhorn,  must  be  our  station.     What 
the  Cleft  Station  is  to  the  Mer  de  Glace,  the 
Riffelhorn  is  to  the  Gorner  glacier  and  its 
tributaries.      From  its  lower  side  the  rock, 
easy  as  it  may  seem,  is  inaccessible.     Here, 


THE  FORMS  OF  WATER 


indeed,  in  1865,  a  fifth  good  man  met  his  ena, 
and  he  also  lies  beside  his  fellow-countrymen 
ia  the  churchyard  of  Zermatt.  Passing  a 
little  tarn,  or  lake,  called  the  Riffel  See,  we 
assail  the  Riffelhorn  on  its  upper  side.  It  is 
capital  rock-practice  to  reach  the  summit; 
and  from  it  we  command  a  most  extraordi- 
nary scene. 

357.  The  huge  and  many-peaked  mass  of 
Monte  Rosa  faces  us,  and  we  scan  its  snows 
fn/m  bottom  to  top.  To  the  right  is  the 
mighty  ridge  of  the  Lyskamm,  also  laden 
with  snow  ;  and  between  both  lies  the  West- 
ertt  G-lacier  of  Monte  Rosa.  This  glacier 
meets  another  from  the  vast  snow-fields  of 
the  Oima  di  Jazzi ;  they  join  to  form  the 
Gorner  glacier,  and  from  their  place  of  junc- 
tion stretches  the  customary  medial  moraine. 
On  this  side  of  the  Lyskamm  rise  two  beau- 
tiful snowy  eminences,  the  Twins  Castor  and 
Pollux  ;  then  come  the  brown  crags  of  the 
Brei thorn,  then  the  Little  :viatteihorn,  and 
then  the  broad  snow-field  of  trie  Theodule, 
out  of  which  cp rings  the  Great  Matterhorn, 
and  which  you  and  I  will  cross  subsequently 
into  Italy. 

o->3.  The  valleys  and  depressions  between 
these  mountains  are  filled  with  glaciers. 
D  )wn  the  flunks  01  the  Twin  Castor  comes 
the  Glacier  des  Jumeaux,  from  Pollux  comes 
the  Schwartze  glacier,  from  the  Breithorn 
the  Trifti  glacier,  then  come  the  Little  Mat- 
terhorn glacier  and  the  Theodule  glacier, 
each,  as  it  welds  itself  to  the  trunk,  carrying 
with  it  its  medial  moraine.  We  can  count 
nine  such  moraines  from  our  present  posi- 
tion. And  to  a  still  more  surprising  degree 
than  on  the  Mer  de  Glace,  we  notice  Hie 
power  of  the  ice  to  yield  to  pressure  ;  the 
broad  neves  being  squeezed  on  the  trunk  of 
the  Goraer  into  white  stripes,  which  become 
ever  narrower  net  ween  their  bounding  mo- 
raines, and  finally  disappear  under  their  own 
shingle. 

859.  On  the  two  main  tributaries  we  also 
notice  moraines  which  seem  in  each  case  to 
rise  from  the  body  of  the  glacier,  appearing 
in  the  middle  of  the  ice  without  any  apparent 
origin  higher  up.  These  at  their  sources 
are  sub-glacial  moraines,  which  have  b^cn 
rubbed  away  from  rocky  promontories  en- 
tirely covered  with  ice.  They  lie  hidden  for 
a  time  in  the  body  of  the  glacier,  and  appear 
at  the  surface  where  the  ice  above  them  has 
been  melted  away  by  the  sun. 

SCO.  This  is  the  place  to  mention  a  notion 
long  entertained  by  the  inhabitants  of  the 
high  Alps,  that  glaciers  possess  the  power  of 
thrusting  out  all  impurities  from  them.  On 
the  Mer  de  Glace  you  and  I  have  noticed 
iargxi  patches  of  clay  and  black  mud  which 
evidently  came  from  the  body  of  the  glacier, 
and  we  can  therefore  understand  how  natural 
was  this  notion  of  extrusion  to  people  unac- 
customed to  close  observation.  But  the 
power  of  the  glacier  in  this  respect  is  in 
reality  the  power  of  the  sun,  which  fuses  the 
ice  above  concealed  impurities,  and,  hke  the 
bodies  of  the  guides  on  the  Glacier  des  Bos- 
sons  (143)  brings  them  to  the  light  of  dav 


361.  On  no  other  glacier  will  you  find  more 
objects  of  interest  than  on  the  Gorner.     Sand 
cones,  glacier-tables,  deep  ice-gorges  cut  by 
streams  and  bridged  fantastically  by  bould- 
ers, moulins,  sometimes  arched  ice-caverns 
of  extraordinary  size  and  beauty.      On  the 
lower  part  of  the  glacier  we  notice  the  par- 
tial disappearance  of  the  medial  moraine  in 
the  crevasses,  ami  its  reappearance  at  the  foot 
of  the  incline.     For  many  years  this  glacier 
was  steadily  advancing  on  the  meadow  in 
front  of  it,  ploughing  up  the  soil  and  over- 
turning the  chalets  in  its  way.    It  now  shares 
in  the  general  reticat  exhibited  during  the 
last  fifteen  years  among  the  glaciers  of  the 
Alps.     As  usual,  a  river,  the  Visp,  rushes 
from  a  vault  at  the  extremity  of  the  Gorner 
glacier. 

§  53.  ANCIENT  GLACIERS  OP  SWITZERLAND, 

362.  You  have  not  lost  the  memory  of  the 
old  moraine,  which  interested  us  so  much  in 
our  first  ascent  from  the  source  of  Ihe  Arvei- 
ron  ;  for  it  opened  our  minds  to  the  fact  that 
at  one  period  of  its  history  the  Mer  de  Glace 
attained  far  greater  dimensions  than  it  now 
exhibits.      Our  experience  since    that  time 
has  enabled  us  to  pursue  these  evidences  of 
ice  action  to  an  extent  of  which  we  had  tiiea 
no  notion. 

363.  Close  to  the  existing  glacier,  for  ex- 
ample,  we  have  repeatedly  seen  the  mountain- 
side laid  bare  by  the  retreat  of  the  ice.     This 
is  especially  conspicuous  just  now,  because 
for  the  last  fifteen  or  sixteen  years  the  glaciers 
of  the  Alps  have  been  steadily  shrinking  ;  so 
that  it  is  no  uncommon  thing  to  see  the  mar- 
ginal rocks  laid  bare  for  a  height  of  fifty, 
sixty,  eighty,  or  even  one  hundred  feet  above 
the  present  glacier.     On  the  rocks  thus  ex- 
posed we  see  the  evident  marks  of  the  slid- 
ing ;  and  our  e}7es  and  minds  have  been  so 
educated  in  the  observation  of  these  appear- 
ances that  we  are  now  able  to  detect,  with 
certainty,  icemarks,  or  moraines,  ancient  or 
modern,  wherever  they  appear. 

364.  But  the  elevations  at  which  we  have 
found  such  evidence  might  well  shake  be-lief 
in    the    conclusions    to    which   they   point. 
Beside  the  Massa  Goige,  at  1000  feet  above 
the  present  Aletsch,  we   found  a  great  old 
moraine.     Descending  the  meadows  between 
the  Be!  Alp  and  Plat  ten,  we  found  another, 
now  clothed  with  grass,  and  bearing  a  village 
on  its  back.     But  I  wish  to  carry  you  to  a 
region  which  exhibits  these  evidences  on  a 
still  grander  and  more  impressive  scale.     We 
have  already  taken  a  brief  flight  to  the  valley 
of  Hasli  and  the  Glacier  of  the  Aar.     Let  us 
make  that  glacier  our  starting-point.     Walk- 
irg  from  it  downward  toward  the  Grimsel, 
we  pass  everywhere  over  rocks  singularly 
rounded,   an  1  fluted,   and    scarred.      These 
appearances  are  manifestly  the  work  of  the 
glacier  in  recent  times.     But  we  approach  the 
Grimsel,    and   at  the  turning  of  the  valley 
stand  before  the  precipitous  granite  flank  of 
the  mountain.     The  traces  of  the  ancient  ice 
are  here  as  plain  as  they  are  amazing.     The 
rocks  are  so  hard  that  not  only  the  fluting 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


129 


and  polishing,  but  even  the  fine  scratches 
'^hich  date  back  unnamable  thousands  of 
years  are  as  evident  as  if  they  had  been  made 
yesterday.  We  may  trace  these  evidences  to 
a  height  of  two  thousand  feet  above  the  pres- 
ent valley  bed.  It  is  indubitable  that  an 
ice-river  of  this  astounding  depth  once  flowed 
through  the  vale  of  Hasli. 

365.  Yonder  is  the  summit  of  the  Siedel- 
h  rn  ;  and  if  we  gain  it  the  Unteraar  glacier 
\v :.'!  lie  like  a  map  below  us.      From  this 
commanding  point  we  plainly  see   marked 
upon  the  mountain-sides  the  height  to  which 
the  ancient  ice  extended.     The  ice  ground 
pail  of  the  mountains  is  clearly  distinguished 
from  the  splintered  crests  which  in  those  dis- 
tant clays  rose  above  the  surface  of  the  glacier, 
and  which  must  have  then  appeared  as  island 
peaks  and  crests  in  the  midst  of  an  ocean  of 
ice. 

366.  We  now  scamper  down  the  Siedelhora, 
get  once  more  into  the  valley  of  Hasli,  along 
which  we  follow  for  more  than  twenty  miles 
the  traces  of  the  ice.     Fluted  precipices,  pol- 
ished slabs,  and  beautifully-rounded  granite 
domes.     Right  and  left  upon  the  mountaiu 
flanks,  at  great  elevations,  the  evidences  ap- 
pear.    We  follow  the  footsteps  of  the  glacier 
to  the  Lake  of  Brientz  ;  and  if  we  prolonged 
our  inquiries,  we  should  learn  that  all   the 
lake  beds  of  this  region,  at  the  tinia  now  re- 
ferred to,  bore  the  burden  of  immense  masses 
of  ice. 

367.  Instead  of  the  vale  of  Hasli,  we  might 
take  the  valley  of  the  Rhone.    The  traces  of 
a  mighty  glacier,  which  formerly  rillei  it, 
may  be  followed  all  the  way  to  Martiguy, 
which  is  0(J  miles  distant  from  the  present 
ice.     At  Martigny  the  Rhone  glacier  was  re- 
inforce t  by  another  from  Mont  Blanc,  and 
the  welJed  masses  moved  onward,  planing 
the  mountains  right  and  left,  to  the  lake  of 
Geneva,    the   basin  of  which   they  entirely 
Billed.     Oilier  evidences  prove  that  the  glacier 
did  not  end  hare,  but  pushed  across  the  low 
country  until  it  encountered  the  limestone 
barrier  of  the  Jura  Mountains. 

§  54.  ERRATIC  BLOCKS. 

368.  What  are  these  other  evidences  ?    We 
have  seen  mighty  rocks  poised  on  the  mo- 
raines of  the  Mer'de  Glace,  and  we  now  know 
that,  unless  they  are  split  and  shattered  by 
the  frost,  these' rocks  will,  at  some  distant 
day,  be  landed  bodily  by  the  Glacier  des  Bois 
in  the  valley  of  Chamouni.     You  have  al- 
ready learned  that  these  boulders  often  reveal 
the  rnin  era  logical  nature  of  the   mountains 
among  which  the  glacier  has  passed  ;   that 
specimens  are  thus  brought  clown  of  a  char- 
acter totally  different  from  the  rocks  among 
•which  they  are  finally  landed  ;  this  is  striK- 
ingly  the  case  with  the  erratic  block*  strandeJ 
along  the  Jura. 

361).  For  the  Jura  itself,  as  already  stated, 
ia  limestone  ;  there  is  no  trace  of  native 
granite  to  be  found  among  these  hills.  Still 
along  the  breast  of  the  mountain  above  the 
town  of  Neufchatel,  and  at  about  800  feet 
above  the  lake  of  Neufchatel,  we  find 


stranded  a  belt  of  granite  boulders  from 
Mont  Blanc.  And  when  we  clear  the  soil 
away  from  the  adjacent  mountain-side,  we 
find  upon  the  limestone  rocks  the  scarrings 
of  the  ancient  glacier  which  brought  the 
boulders  here. 

370.  The    most    famous  of    these  rocks, 
called  the  Pierre  il  Bot,  measures  50  feet  in 
length,  40  in  height,  and  20  in  width.     Mul- 
tiplying  these   three  numbers  together,  we 
obtain  40,000  cubic  feet  as  the  volume  of  the 
boulder. 

371.  But  this  is  small  compared  to  som« 
of  the  rocks  which  constitute  the  freight  of 
even  recent  glaciers.     Let  us  visit  another  of 
them.      We  have  already  been  to  Stalden, 
where  the  valley  divides  into  two  branches, 
the  right  branch  running  to  St.  Nicholas  and 
Zermatt,   and  the  left  one  to  Saas  and  the 
Monte  Moro.     Three  hours  above  f:iaas  we 
come  upon  the  end  of  the  Allelein  glacier, 
not    filling    the    main   valley,    but    thrown 
athwart  it  so  as  to  sto-p  its  drainage  like  a  dam. 
Above  this  ice-dam  we  have  the  Matt  mark 
Lake,  and  at  the  head  of  the  lake  a  small  inn 
well  known  to  travellers  over  the  Monte  Moro. 

372.  Close  to  ihis  inn  is  the  greatest  bould- 
er that  we   have  ever  seen.      It  measures 
240,000  cubic  feet.      Looking  across  the  val- 
ley we  notice  a  glacier  with  its  present  end 
half  a  mile  from  the  boulder.     The  stone,  I 
believe,  is  serpentine,  and  were  you  and  I  to 
explore  the  Schwartzberg  glacier  to  its  upper 
fastnesses,  we  should  find  among  them*  the 
birthplace  of  this  gigantic  stone.     Four-and- 
forty  years  ago,  when  the  glacier  reached  the 
place  now  occupied  by  the  boulder,  it  landed 
there  its  mighty  freight,  and  then  retreated. 
There  is  a  second  ice-borne  rock  at  hand, 
which  would  be  considered  vast  were  it  not 
dwarfed  by  the  aspect  of  its  huger  neighbor. 

373.  Evidence  of  this  kind  might  be  multi. 
plied  to  any  extent.     In  fact,  at  this  moment, 
distinguished  men,  like  Professor  Favre  of 
Geneva,  are  determining  from  the  distribu- 
tion of  the  erratic  blocks  the  extent  of  the 
ancient    glaciers  of   Switzerland.      It  was, 
however,  an  engineer  named  Venctz  that  first 
brought  these  evidences  to    light,  and  an- 
nounced to  an  incredulous   \vorld  the  vast 
extension  of  the  ancient   ice.     M.   Agassiz 
afterward   developed  and    wonderfully  ex- 
panded the  discovery.     Pehaps  the  most  in- 
teresting observation  regarding  ancient  gla- 
ciers is  that  of  Dr.  Hooker,  who,  during  a 
recent  visit  to  Palestine,  found  the  celebrated 
Cedars  of   Lebanon   growing  upon  ancient 
moraines. 

§  55.  ANCIENT  GLACIERS  OF  ENGLAND, 
IRELAND.  SCOTLAND,  AND  WALES. 

374.  At  the  time  the  ice  attained  this  extra- 
ordinary development   in    the   Alps,    many 
other  portions  of  Europe,  where  no  glaciers 
now  exist,  were  covered  with  then).     In  the 
Highlands  of  Scotland,  among  the  mount.uins 
of  England,  Ireland,  and  Wales,  the  ancient 
glaciers  have  written  their  story  as  plainly 
as  in  the  Alps  themselves      I  should  like  te 
wander  with  you  through  Borrodale  in  Cuia- 


124 


THE  FOKMS  OF  WATER. 


berland,  or  through  the  valleys  near  Beth- 
gellert  in  Wales.  Under  all  tlie  beauty  of  the 
present  scenery  we  should  discover  the  me- 
morials of  a  time  when  the  whole  region  was 
locked  in  the  embrace  of  ice.  Professor 
Ramsay  is  especially  distinguished  by  his 
writings  oil  the-  ancient  glaciers  of  Wales. 

875.  We  have   made   the  acquaintance  of 
the  Keeks  ot  Magillicuddy  as  the  great  con- 
densers of  Atlantic  vapor.     At  the  time  now 
referred  to,  this  moisture  did  not  fall  as   soft 
and    fructifying    rain,   but  as  snow,    which 
formed  the  nutriment  of  great  glaciers.     A 
chain  of  lakes  now  constitutes  the  chief  at- 
traction of  Killarney,  the  Lower,  the  Middle, 
and  the  Upper  Luke.      Let  us  suppose  our- 
selves rowing  toward  the  head  of  the  Upper 
Lake  with  the  Purple  Mountain  to  our  left. 
Remembering  our  travels  ill  the  Alps,  you 
would    infallibly  call   my  attention    to    the 
planing  of  the  rocks,  and  declare  the  action 
to  be    unmistakably  that  of   glaciers      With 
our  attention  thus  sharpened,  we  land  at  the 
heal   of  the   lake,  and  walk  up  the    Black 
Valley  to  the  bas«  of  Magillicuddy 's   Keeks. 
Your  conclusion  would   be,  that    this   valley 
tells  a  tale  as  wonderful  as  that  of  Hasli. 

876.  We  reaeli   our  boat  and  row   home- 
ward along    the   Upper   Lake.      Its  islands 
now  possess  a  new  interest  for  us.     Some  of 
them  are  bare,  others  are  covered  wholly  or 
in  part  with  luxuriant  vegetation  ;  but   both 
the  naked  and  clothed  islands  are  glaciated. 
The  weathering  of  ages  has  not  altered  their 
forms ;    there   are    the    Cannon     Rock,   the 
Giant's   Coffin,   the   Man-of-War,  all   sculp- 
tured as  if  tlie  chisel  had  passed  over  them  in 
our  own  lifetime.     These  lakes,  now   fringed 
with  tender  woodland  beauty,  were  all  occu- 
pied by  the  ancient  ice.      It  has  disappeared, 
and    seeds    from    other    regions   have   been 
wafted  thither  to  sow  the  trees,  the  shrubs, 
the  ferns,  and  the  grasses  which  now  beau- 
tify Killarney.     Mun  himself,  they  say,  lias 
made  his  appearance  in  the  world  since  that 
time  of  ice  ;  but  of  the  real  period  and  manner 
of  man's  introduction  little  is  professed  to  be 
known  since,  to  make  them  square  with  sci- 
ence, new. meanings  have  been  found  for  the 
beautiful  myths  and  stories  of  the  Bible. 

377.  It  is  the  nature  and  tendency  of  the 
human  mind  to  look  backward  and  lor  ward  ; 
to  endeavor  to  restore  the  past  and  predict 
the  future.       Thus  endowed,  from  data   pa- 
tiently and    painfully    won,  we    recover   in 
idea  a  state  of  things  which   existed   thou- 
sands, it  may  be  millions,  of  years  before  the 
history  of  the  human  race  began. 

§  56.  THE  GLACIAL  Erocn. 

378.  This  period  of  ice-extension  lias  been 
named   the    Glacial  Epoch.      In   accounting 
for  it  great  minds  have  fallen  into  grave  er- 
rors, as  we  shall  presently  see. 

379.  The  substance  on  which  we    have 
thus  far  been  working  exists  in  three  differ- 
ent states :  as  a  solid  in  ice  ;  as  a  liquid  in 
water ;    as  a  gas  in  vapor.      To  cause  it  to 
pass  from  one  of  these  states  to  the  next  fol- 
lowing one,  heat  is  necessary. 


330.  Dig  a  hole  in  the  ice  of  the  Mer  do 
Glare  in  summer,  and  p  ace  si  thermometer 
in  the  hole  ;  it  will  suuid  ut  32°  Fa.hr.  Dip 
your  thermometer  into  one  of  the  glacier 
streams;  it  will  still  mark  32°.  The  water  is 
therefore  as  cold  us  ice. 

381.  Hence  the  whole  of  the.  heat  j-oured 
by  the  sun  upon  the  glacier,  and  which  has 
I  een  absorbed  by  the  glacier,  is  expended  ia 
simply  liquefying  the  ice,  and  not  in  render- 
ing either  ice  or  water  a  single  degree  warmer. 

382.  Expose  water  to  a  lire  ;    it    becomes 
hotter  for  a  time.      It  boils,  and  from  that 
moment  it  ceases  to  g  t  h<  tter.     After  it  has 
begun  to  boil,  all  the  hrni  communicated  by 
the  fire  is  carried  away  by  the  steam,  t?iouyh 
the  steam  itself  is  not  the  least  fraction  -fa  de- 
gree hotter  than  the  wafer. 

883.  In  fact,  simply  to  liquefy  ice  a  large 
quantity  of  heat  is  necessary,  and  to  vaporize 
water   a   still  larger  quantity    is    necessary. 
And  inasmuch  as  thi>  heat   does  not   render 
the  water  warmer  than  the  ice,  nor  the  steam 
warmer  than    the  water,  it  was  at,   one   time 
supposed  to  be  hidden  in  the  water  and  in  the 
Fleam.     And  it  was   therefore   called   late/it 
heat. 

884.  Let  us  ask  how   much  heat  must  the 
sun    expend    in    order   to   convert   a    pound 
weight   of    the    tropical   ocean    into    \upor? 
This  problem  has  been  ticcurately  solved   by 
experiment.     It  would  require  in  round  num- 
bers 1000  times  the  amount  of  heat  necessary 
to  raise  one   pound  of   water  one   degree  in 
temperature. 

383.  But  the  quantity  of  heat  which  wou'.d 
raise   the  tempi  rature  of   a  pound  of  water 
one  degree  would  raise  the  tempt  rature  of  a 
pound  of  iron  ten  degrees.     This  has   been 
also   proved  by  experiment.     IK-nee  to  con- 
vert  one   pound    of  the  tropical    ocean   into 
vapor  i he  sun  must  expend  10,000  times  as 
much  heat  as  would  laise  one  pound  of  iron 
one  degree  in  temperature. 

386.  This  quantity  of  heat  wr.uld  raise  the 
temperature  of   5  Ibs.  of  iron   20UO   degrees, 
which   is   the  fusing   point   of  cast  iron  ;   at 
this  temperature  die  metal  would  not  only  be 
white  hot,  but  would  be  passing  into  the  mol- 
ten condition. 

387.  Consider  the  conclusions  at  which  we 
have   now    arrived.      For    every    pound   of 
tropical  vapor,  or  for  every  pound  of  Alpine 
ice   produced  by  the  congelation  of  that  va- 
por, an   amount  of   heat  has  been  expended 
by  the  sun  sufficient  to  raise  5  Ibs.  of  cast- 
iron  to  its  melting-point. 

388.  It  would  not  be  difficult  to  calculate 
approximately    the   weight  of  the    Mer   do 
Glace  and  its  tributaries — to  say,  fbr  exam- 
ple, that   they  contained  so  many  millions  of 
millions  of  tons  of  ice  and  snow.     Let  the 
place  of  the  ice  be  taken  by  a  mass  of  white- 
hot  iron  of  quintuple  the  weight ;  with  such 
a  picture    before  your  mind  you  get  some 
notion  of  the  enormous  amount  of  heat  paid 
out  by  the  sun  to  produce  the  present  glacier. 

389.  You  must  think  over  this,  until  it  is 
as  clear  as  sunshine.     For  you   must  never 
henceforth  fall  into  the  error  already  referred 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


125 


to,  and  which  has  entangled  so  many.  So 
natural  was  the  association  of  ice  and  cold, 
that  even  celebrated  men  assumed  that  all 
that  is  needed  to  produce  a  great  extension 
of  our  glaciers  is  a  diminution  of  the  sun's 
temperature.  Had  they  gone  through  the 
foregoing  reflections  and  calculations,  they 
would  probably  have  demanded  more  heat 
instead  of  less  for  the  production  ot  a  "  gla- 
cial epoch."  What  they  really  needed  were 
condensers  sufficiently  powerful  to  'congeal 
the  vapor  generated  by  the  heat  of  the  sun. 

§  57.  GLACIER  THEORIES. 
390.  You  have  not  forgotten,  and  hardly 
ever  can  forget,  our  climbs  to  the  Cleft  Sta- 
tion. Thoughts  were  then  suggested  which 
we  have  not  yet  discussed.  We  saw  the 
branch  glaciers  coming  down  from,  their 
neves,  welding  themselves  together,  pushing 
through  Trelaporte,  and  afterward  moving 
through  the  sinuous  valley  of  the  Mer  de 
Glace.  These  appearances  alone,  without 
taking  into  account  subsequent  observa- 
tions, were  sufficient  to  suggest  the  idea  that 
glacier  ice,  however  hard  and  brittle  it  may 
appear,  is  really  a  viscous  substance,  resem- 
bling treacle,  or  honey,  or  tar,  or  lava. 

§  53.  DILATATION  AMD  SLIDING  THEORIES. 

891.  Still  this  was  not  the  notion  expressed 
by  the  majority  of  writers  upon  glaciers. 
Scheuchzer  of  Zurich,  a  great  naturalist,  vis- 
ited the  glaciers  in  1705,  nnd  propounded  a 
theory  of  their  motion.  Water,  he  knew, 
expands  in  freezing,  nnd  the  force  of  expan- 
sion is  so  great  that  thick  bomb-shells  filled 
with  water,  and  permitted  to  freeze,  are,  as 
we  know  (812),  shattered  to  pieces  by  the  ico 
within.  Scheuchzer  supposed  that  the  wa- 
ter in  the  fissures  of  the  irlacier-s,  freezing1 
there  and  expanding  with  resistless  force, 
was  the  power  which  urged  the  glacier 
downward.  He  added  to  this  theory  other 
notions  of  a  less  scientific  kind. 

392.  Many   years  subsequently,  De  Char- 
pentier  of  Bex  renewed  and  developed    this 
theory  with   such  ability  and   completeness 
that    it   was    long    known   as   Ciiarpemier'a 
Theory  of  Dilatation.     M.  Agassiz  lor  a  time 
espoused  this  theory,  and  it  was  aij«o  more  or 
less  distinctly  held  by   other  wriieis.     The 
glacier,  in  fact,  was  considered  to  be  a  mag- 
azine of  cold,  capable  of  freezing  all  water 
percolating    through     it.     The     theory    was 
abandoned  when  this  notion  of  glacier  cold 
was  proved  by  M.  Agassiz  to  be  untenable. 

393.  In  17b'0,  Altmanu  and  Griiner  pro- 
pounded the   view    that  glaciers  moved  by 
•sliding  over  their  beds.     Nearly  lorty  years 
subsequently,  this  notion  was  revived  by  De 
Saussure,  and  it  has  therefore   been  called 
"De  Saussure's   Theory,"  or   the   "Sliding 
Theory,"  of  glacier  motion. 

394.  There  was,  however,  but  little  reason 
to  connect  the  name  of  De  Saussure  with  this 
or  any  other  theory  of  glaciers.     Incessantly 
occupied  in   observations   of  another   kind, 
this  celebrated  man  devoted  very  little  time 
or  thought  to  the  question  of  glacier  motion. 


What  he  has  written  upon  the  subject  reads 
less  like  the  elaboration  of  a  theory  than  the 
expression  of  an  opinion. 

§  59.  PLASTIC  THEORY. 

395.  By  none  of  these  writers  is  the  prop- 
erty of  viscosity  or  plasticity  ascribed  to  gla- 
cier ice  ;  the  appearances  of  many  glaciers 
are,  however,  so  suggestive  of  this  idea  that 
we  may  be  sure  it  would  have  found  Ujore 
frequent  expression,  were  it  not  in  such  ap- 
parent contradiction  with  our  every  day  ex- 
perience of  ice. 

396.  Still  the  idea  found  its  advocates.     In 
a  little  book,  published  in  1773,  and  entitled 
"Picturesque    Journey   to    the  Glaciers  of 
Savoy,"    Bordier    of    Geneva    wrote    thus  : 
"  It  is  now  time  to  look  at  all  these  objects 
with   the  eyes  of   reason  ;   to  study,  in  the 
first  place,  the  position  and  the  progression 
of  glaciers,  and  to  seek  the  solution  rf  their 
principal  phenomena.     At  the  first  aspect  of 
the  ice-mountains  an  observation  presents  it- 
self, which  appears  sufficient  to  explain  all. 
It  is  that  the  entire  mass  of  ice  is  connected 
together,  and  presses  from  above  downward 
after  the  manner  of  fluids.     Let  us  then  re- 
gard the  ice,  not  as  a  mass  entirely  rigid  and 
immobile,     but  as  a    heap    of    coagulated 
matter,  or  as  softened  wax,  flexible  and  duc- 
tile to  a  certain  point."     Here  probably  for 
the  first  time  the  quality  of  plasticity  is  as- 
cribed to  the  ice  of  glaciers. 

397.  To  us,  familiar  with  the  aspect  of  tho 
glaciers,  it  must  seem  strange  that  this  idea 
once  expressed  did  not  at  once  receive  recog- 
nition and  development.     But  in   those  early 
days  explorers  were  few,  and  the   "  Pictur- 
esque Journey"  probably  hut  little  known,, 
so  that  the  notion  of  plasticity  lay  dormant 
for  more  than  half  a  century.     But  Bordier 
was  at  length   succeeded   by  a  man  of  far  • 
greater  scientific  £rapp  and  insight  than  him- 
self.    This  was  Rendu,  a  Catholic  priest  and, 
canon  when  he  wrote,  and  afterward  Bishop  < 
of  Annecy.     In  1841  Rendu  laid  before  the 
Royal  Academy   of   Sciences   of    Savoy   his- 
"Theory  of  the  Glaciers  of  Savoy,"  a  con- 
tribution forever  memorable  in   relation  to; 
this  subject. 

398.  Rendu  seized  the  idea  of  glacier  plas- 
ticity with  great  power  and  clearness,  and-; 
followed  it   resolutely   to  its   consequences. 
It  is  not  known  that  ho  had  ever  seen  the 
work  of  Bordier  ;   probably  not,  as  he  never- 
mentions  it.     Let  me  quote  for  you  some  of 
Rendu's  expressions,  which,  however,  fail  to 
give  an  adequate  idea  of  his  insight  and -pre- 
cision of  thought:  "Between    the  Mer  de 
Glace  and  a  river  there  is  a  resemblance  BO 
complete  that  it  is  imposiible  to  find  in  the. 
glacier  a  circumstance  which  does  not  exist 
in  the  river.     In  currents  of  water  the  mo- 
tion is  not  uniform  either  throughout  their 
width  or  throughout  their  depth.     The  fric- 
tion of  the  bottom  and  of  the  sides,  with  the 
action  of  local  hindrances,  causes  the  motion 
to  vary,  and  only  toward  the  middle  of  the 
surface  do  we  obtain  the  full  motion." 

399.  This  reads  like  a  prediction  of  what 


126 


THE  FORMS  OF  WATER 


has  since  been  established  by  measurement. 
Looking  at  the  glacier  of  Mont  Dolent,  which 
resembles  a  sheaf  in  form,  wide  at  both  ends 
and  narrow  in  the  middle,  and  reflecting  tbat 
*he  upper  wide  part  had  become  narrow,  and 
the  narrow  middle  part  again  wide,  Rcndu 
observes,  "  There. is  a  multitude  of  facts 
which  seem  to  necessitate  the  belief  that  gla- 
eiar  ice  enjoys  a  kind  of  ductility  which  en- 
ables if,  to 'mould  itself  to  its  locality,  to  thin 
out,  to  swell,  and  to  contract  as  if  it  were  a 
soft  paste." 

400.  To  fully  test  his  conclusions,  Rendu 
required  the  accurate  measurement  of  glacier 
motion.  Had  he  added  to  his  other  endow- 
ments the  practical  skill  of  a  land-surveyor, 
he  would  now  be  regarded  as  the  prince  of 
glacialists.  As  it  was  he  was  obliged  to  be 
content  with  imperfect  measurements.  In 
one  of  his  excursions  he  examined  the  guides 
regarding-  the  successive  positions  of  a  vast 
rock  which  he  found  upon  the  ice  close  to 
the  side  of  the  glacier.  The  mean  of  five 
years  gave  him  a  motion  for  this  block  of  40 
feet  a  year. 

401.'  Another  block,  the  transport  of  which 
he  subsequently  measured  more  accurately, 
gave  him  a  velocity  of  400  feet  a  year.  Note 
his  explanation  of  this  discrepancy  :  '•*  The 
enormous  difference  of  these  two  ol)eerva- 
tions  arises  from  the  fact  that  one  block 
stood  near  the  centre  of  the  glacier,  which 
moves  most  rapidly,  while  the  other  stood 
noar  the  side,  where  the  ice  is  held  back  by 
friction. "  So  clear  and  definite  were  Rcndu's 
ideas  of  the  plastic  motion  of  glaciers,  that 
had  the  question  of  curvature  occurred  to 
him,  I  entertain  no  doubt  that  he  would 
have  enunciated  beforehand  the  shifting  of 
the  point  of  maximum  motion  from  side  to 
side  across  the  axis  of  the  glacier  (§  25). 

402.  It  is  right  that  you  should  know  that 
scientific  men  do  not  always  agree  in  their 
estimates  of  the  comparative  value  of  facts 
?md  ideas  ;  and  it  is  especially  right  that  you 
should  know  that  your  present  tutor  attaches 
a  very  high  value 'to  ideas  when  they  spring 
from  the  profound  and  persistent  pondering 
of  superior  minds,  and  are  not,  as  is  too 
often  the  case,  thrown  out  without  the  war- 
rant of  either  deep  thought  or  natural  capac- 
ity.    It  is  because  I  believe  Rendu's  labors 
fulfil  this  condition  that  I  ascribe  to  them  so 
high  a  value.     But  when  you  become  older 
and  better  informed,  you  may  differ  from 
i-iie  ;  and  I  write  these  words  lest  you  should 
too  readily  accept  my  opinion  of  Rendu. 
Judge  me,'  if  you  care  to  do  so,  when  your 
knowledge  is  matured.     I  certainly  shall  not 
!  ear  your  verdict. 

403.  But,  much  as  I  prize  the  prompting 
idea,  and  thoroughly  as  I  believe  that  often 
in  it  the  force  of  genius  mainly  lies,  it  would, 
in  my  opinion,  be  an  error  of  omissioa  of  the 
gravest  kind,  and  which,  if  habitual,  would 
insure  the  ultimate  decay  of  natural  knowl- 
•  «lge,  to  negL-ct  verifying  our  ideas,  and  giv- 
ng  them  outward  reality  and  substance  when 
'.lie  means  of  doing  so  are  at  hand.     In  sci- 
jnce,  thought,  as  far  as  possible,  ought  to  foe 


wedded  to  fact,  This  was  attempted  by 
Rendu,  and  in  great  part  accomplished  by 
Agassiz  and  Forbes. 

§  GO.  Viscous  THEORY. 

404.  Here  indeed  the  merits  of  the  distin- 
guished Racialist  last  named  rise  conspicu- 
ously to  view.     From  the  able  and  earnest 
advocacy    of  Professor  Forbes,   the  public 
knowledge  of  this  doctrine  of  glacial  plastic- 
ity is  aUnost  wholly  derived.     He  gave  the 
doctrine  a  more  distinctive  form  ;   he  first 
applied  the  term  viscous  to  glacier  ice,  and 
sought  to  found  upon  precise  measurements 
a  "viscous  Theory"  of  glacier  motion. 

405.  I  am  here  obliged  to  state  facts  in 
their  historic  sequence.     Professor   Forbes 
when  he  began  his  investigations  was  ac- 
quainted with  the  labors  of  Rendu.     In  his 
earliest  work  upon  the    Alps  he  refers  to 
those  labors  in  terms  of  flattering  recogni- 
tion.    But  though  as  a  ma  te:-  of  f act  lien- 
du's  ideas  were  there  to  prompt  him,  it  would 
be  too  much  to  say  that  he  needed  their  in- 
spiration.    Had  Rcndu  not   preceded   him, 
he  might  none  the  less  have  grasped  the  idea 
of  viscosity,  executing  his  measurements  and 
applying  his  knowledge  to  maintain  it.     Be 
that  as  it  may,  the  appearance  of  Professor 
Forbes  on  the  Unteraar  glacier  in  1841,  and 
on  the  Mer  cle  Glace  in  1842,  and  his  labors 
then  and  subsequently,  have  given    him  a 
name  not  to  be  forgotten  in  the  scientific  his- 
tory of  glaciers. 

406.  The  theory  advocated  by  Professor 
Forbes  was  enunciated  by  himself  in  these 
words  :    "A  glacier  is  an  imperfect  fluid,  or 
viscous  body,  which  is  urged  down  slopes  of 
certain  inclination  by  the  natural  pressure  of 
its  parts."      In  1773  Bordier  wrote  thus: 
"As  the  glaciers  always  advance  upon  the 
plain,  and  never  disappear,  it  is  absolutely 
essential  that  new  ice  shall  perpetually  take 
the  place  of  that  which  is  melted  :  it  must 
therefore  be  pressed  forward  from   above. 
One  can  hardly  refuse  then  to  accept  the  as- 
tonishing truth,  that  this  vast  extent  of  haul 
and  solid  ice  moves  as  a  single  piece  down- 
ward."    In  the  passage  already  quoted  he 
speaks  of  the  ice  being  pressed  as  a  fluid 
from  abore.     Tirap  constitute,   I    believe, 
Bordier's  contributions  to  this  subject.     The 
quotations  show  his   sagacity  at  an  early 
date  ;    but,    in  point  of    completeness,    his 
views  are  not  to  be  compared  with  those  of 
Rendu  nnd  Forbes. 

407.  I  must  not  omit  to  state  here  that 
though  the  idea  of  viscosity  has  not  been  es- 
poused by  M.  Agassiz,  his  measurements, 
and  maps  of  measurements,  on  the  Unteraar 
glacier  have  been  recently  cited  as  the  most 
clear  and  conclusive  illustrations  of  a  quality 
which,  at  all  events,  closely  resembles  vis- 
cosity. 

408.  But  why,   with  proofs  before  him 
mem;  copious  and  characteristic  than  those  of 
any  other  observer,  does  M.  Agassiz  hesitate 
to  accept  the  idea  of  viscosity  as  applied  to 
ice  ?    Doubtless  because  he  believes  the  no- 
tion to  be  contradicted  by  our  cvery-day  ex- 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


perience  of  the  substance. 

409.  Take  a  mass  of  ice  ten  or  even  fifteen 
cubic  feet  in  volume  ;  drpw  a  saw  across  it 
to  a  depth  of  half  an  inch  or  an  inch  ;  and 
strike  a  pointed  pricker,  not  thicker  than  a 
very  small  round  tile,  into  the  groove  ;  the 
substance  will  split  from  top  to  bottom  with 
a  clean  crystalline    fracture.     How  is  this 
brittleness  to  be;  reconciled  with  the  notion 
of  viscosity  ? 

410.  We  have,  moreover,  been  upon  the 
glacier  and  have  witnessed  the  birth  of  cre- 
vasses.    We  have  seen*  them  beginning  as 
narrow  cracks  suddenly  formed,  days  being 
iL'Ljuired  to   open  them  a  single  inch.     In 
many  glaciers  fissures  may  be  traced  narrow 
and  profound  for  hundreds  of  yards  through 
the  ice.     What  does  this  prove?    Did  the 
ice  possess  even  a   very  small  modicum  of 
that  power  of  stretching,  which  is  character- 
istic of  a  viscous  substance,  such  crevasses 
coul'i  not  be  formed. 

411.  Slill  it  is  undoubted  that  the  glacier 
moves  like  a  viscous  body.     The  centre  ilows 
past  the  sides,  the  top  flows  over  the  bottom, 
and  the  motion  through  a  curved  valley  cor- 
responds to  fluid    motion.     Mr.   Mathews, 
Mr.  Froude,  and  above  all  Signer  Bianconi, 
h;ive,  moreover,  recently  made  experiments 
on  ice  which  strikingly  illustiate  the  flexibil- 
ity  of    the   substance.     These    experiments 
merit,  und  will  doubtless  receive,  lull  atten- 
tion at  a  future  time. 

§  61.  REGELATIOX  THEORY. 

412.  I  will  now  describe  to  you  an  attempt 
that  has  been  made  of  late  years  to  reconcile 
the  brittleness  of  ice  with  its  motion  in  gla- 
ciers.    It  is  founded   on  the    observation, 
made  by  Mr.  Faraday  in  1850,  that  when 
two  pieces  of  thawing  ice  are  placed  to- 
gether they  freeze  together  at  the  place  of 
contact. 

•ii:».  This  fact  may  not  surprise  ^^d  ;  still 
it  ;U  (.idsed  Mr.  Faraday  and  others,  and 
in  (i  or  very  great  distinction  in  science  have 
differed  in  their  interpretation  of  the  fact. 
The  difficulty  is  to  explain  where,  or  how,  in 
ice  already  thawing  the  cold  is  to  be  found 
requisite  to  freeze  tne  film  of  water  between 
the  two  touching  surfaces. 

414.  The  word  Regelaiion  was  proposed  by 
Dr.  Hooker  to  express  the  freezing  together 
of  t\vo  pieces  of  thawing  ice  observed  by 
Faraday  ;  and  the  memoir  in  which  the  term 
was  first  used  was  published  by  Mr.  Huxley 
an;!  ilr.  Tyncla'.l  in  the  Philosophical  Trans- 
actions for  I8,"i7. 

415.  The  fact  of  regulation,  and  its  appli- 
cation irrespective  of  the  cause  of  rcgelation, 
may  be  tnus    illustrated  :     Saw   two  slabs 
from  a  block  of  ice,  and  bring  their  flat  sur- 
faces into  contact,  ;  the}'  immediately  freeze 
together.     Two  platen  of  ice,  laid  one  upon 
the   other,    with    flannel    round  them  over 
night,  ate  sometimes  so  firmly  frozen  in  the 
morning  that  they  will   rather   break   else- 
whei'e  Uum  along  their  surtV.ee  of  junction. 
If  you  erter  one  of  the  dripping  ice-eaves  of 
Switzerland,  you  have  only,  to  press  for  a 


'moment  a  slab  of  ice  against  the  roof  of  tLe 
uave  to  cause  it  to  freeze  there  and  stick  to 
•tfhe  roof. 

416.  Place  a  number  of  fragments  of  ice 
in  a  basin  of  water,  and  cause  them  to  touch 
each  other  ;  they  freeze  together  where  they 
touch.     You  can  form  a  chain  of  such  frag- 
ments ;  and  then,  by  taking  hold  of  one  end 
of  the  chain,  you  can  draw  the  whole  series 
after  it.     Chains  of  icebergs  are  sometimes 
formed  in  this  way  in  the  Arctic  seas. 

417.  Consider  what  follows  from  these  ob- 
servations.    Snow  consists  of  small  particles 
of  ice.     Now  if  by  pressure  we  squeeze  out 
the  air  entangled  in  thawing  snow,  and  bring 
the  little  ice-granules  into  close  contact,  they 
may  be  expected  to  freeze  together  ;  and  if 
the  expulsion  of  the  air  be  complete,  the 
squeezed  snow  may  be  expected  to  assume 
the  appearance  of  compact  ice. 

418.  We  arrive  at  this  conclusion  by  rea- 
soning ;  let  us  now  test  it  by  experiment, 
employing  a  suitable  hydraulic  press,  and  a 
mould  to  hold  the  snow.     In  exact  accord- 
ance with  our  expectation,  we  convert  by 
pressure  the  snow  into  ice. 

419.  Place  a  compact  mass  of  ice  in  a 
proper  mould,  and  subject  it  to  pressure.     It 
breaks  in  pieces  ;  squeeze  the  pieces  forcibly 
together  ;  they  reunite  by  regelation,  and  a 
compact  piece  of  ice,   totally  different   in 
shape  from  the  first  one,  is  taken  from  the 
rress.  To  produce  this  effect  the  ice  must  bo 
in  a  thawing  condition.     When  its  tempera- 
ture is  much  below  the  melting-point  it  is 
crashed    by    pressure,  not    into  a  pellucid 
mass  of  another  shape,    but  into  a  white 
powder. 

430.  By  means  of  suitable  moulds  you 
may  in  this  way  change  the  shape  of  ice  to 
any  extent,  turning  out  spheres,  and  cups; 
and  rings,  and  twisted  ropes  of  the  sub- 
stance ;  the  change  of  form  in  these  cases 
being  effected  through  rude  fracture  and  re- 
gelation. 

421.  By  applying  the  pressure  carefully, 
rude  fracture  may  be  avoided,  and  the  ice 
compelled  slowly  to  change  its  form  as  if  it 
were  a  plastic  body. 

422.  Now  our  first  experiment  illustrates 
the  consolidation  of  the  snows  of  the  higher 
Alpine  regions.     The  deeper  layers  of  tha 
neve  have  to  bear  the  weight  of  all  above 
them,  and  are  thereby  converted  into  more  or 
less  perfect  ice.     And  our  last  experiment 
illustrates  the  changes  of  form  observed  upon 
the  glacier,  where,  by  the  slow  and  constant 
application   of  pressure,   the   ice  gradually 
moulds  itself  to  the  valley  which  it  fills. 

423.  In  glaciers,  however,  we  have  also 
ample  illustrations  of  rude  fracture  and  re-ge- 
lation.    The  opening  and  closing  of  crevasses 
illustrate  this.     The  glacier  is  broken  on  the 
cascades  and  mended  at  their  bases.     When 
two  branch  glaciers  lay  their  sides  together, 
the  regelatiou  is  so  firm  that  they  begin  im- 
mediately to  flow  in  the  trunk  glacier  as  a 
single  stream.     The  medial  moraine  gives  n» 
indication  by  its  slowness  of  motion  that  it  ib 
derived  from  the  sluggish  ice  of  the  sides  o€ 


128 


THE  FORMS  OF  WA^ER 


the  branch  glaciers. 

424.  The  gist  of  the  Revelation  Theory  is 
that  the  ice  of  glaciers  changes  its  form  and 
preserves  its  continuity  under  pressure -which 
keeps  its  particles  together.  But  when  sub- 
jected to  tension,  sooner  than  stretch  it  breaks, 
and  behaves  no  longer  as  a  viscous  body. 

§  62.  CAUSE  OF  REGELATION. 
436.  Here  the  fact  of  regelation  is  applied 
to  explain  the  plasticity  of  glacier  ice,  no 
attempt  being  made  to  assign  the  cause  of 
regelation  itself.  They  are  two  entirely  dis- 
tinct questions.  But  a  little  time  will  be 
•well  spent  in  looking  more  closely  into  the 
cause  of  regelation.  You  may  feel  some 
surprise  that  eminent  men  shoukl  devote  their 
attention  to  so  small  a  point,  but  we  must 
not  forget  that  in  nature  nothing  is  small. 
Laws  and  principles  interest  the  scientific 
student  most,  and  these  may  be  as  well  illus- 
trated by  small  things  as  by  large  ones. 

426.  The  question  of  regelation  immediate- 
ly connects  itself  with  that  of  "  latent  heat," 
already  referred  to  (383),  but  which  we  must 
now    subject  to    further  examination.      To 
melt  ice,  as  already  stated,  a  large  amount 
of  heat  is  necessary,  and  in  the  case  of  the 
glaciers  this  heat  is  furnished  by  the  sun. 
Neither  the  ice  so  melted  nor  the  water  which 
results  from  its  liquefaction  can  fall  below 
32°  Fahrenheit.    The  freezing-point  of  water 
and    the  melting-point  of    ice  touch  each 
other,  as  it  were,  at  this   temperature.     A 
hair's-breadth  lower  water  freezes  ;  a  hair's- 
breadth  higher  ice  melts. 

427.  But  if  the  ice  could  be  caused  to  melt 
without  this  supply  of  solar  heat,  a  tempera- 
ture lower  than  that  cf  ordinary  thawing  ice 
would  result.       When    gnow    and    salt,  or 
pounded  ice  and  salt,  are  mixed  together,  the 
salt  causes  the  ice  to  melt,  and  in  this  way  a 
cold  of  20  or  30  degrees  below  the  freezing- 
point  may  be  produced.     Here,  in  fact,  the 
ice  consumes  its  own  warmth  in  the  work  of 
liquefaction.     Such  a  mixture  of  ice  and  salt 
is  called  "  a  freezing  mixture." 

428.  And  if  by  any  other  means  ice  at  the 
temperature    of   32°*  Fahrenheit    could    be 
liquefied  without  access  of  heat  from  with- 
out, the  water  produced  would  be  colder  than 
the  ice.     Now  Professor  James  Thomson  has 
proved  that  ice  may  be  liquefied    by  mere 
pressure,  and  his  brother.  Sir  William  Thom- 
son, has  also  shown  tbat  water  under  press- 
ure requires  a  lower  temperature  to  freeze  it 
than  when  the  pressure  is  removed.     Pro- 
fessor Mousson  subsequently  liquefied  large 
masses  of  ice  by  a  hydraulic  press  ;  and  by 
a  beautiful  experiment  Professor  Helmholtz 
has  proved  that  water  in  a  vessel  from  which 
the   air  has  been  removed,    and   which   is 
therefore  relieved  from  the  pressure  of  the 
atmosphere,  freezes  and   forms  ice-crystals 
when  surrounded  by  melting  ice.     All  these 
facts  are  summed  up  in  the  brief  statement 
that  the  freezing-point  of  water  is  lowered  b$ 
pressure. 

429.  For  our  own  instruction  we  may  pro- 
duce the  liquefaction  of  ice  by  -pressure  in 


the  following  way :  You  remember  the 
beautiful  flowers  obtained  when  a  sunbeam 
is  sent  through  lake  ice  (§  11),  and  you  have 
not  forgotten  that  the  flowers  always  foirn 
parallel  to  the  surface  of  freezing.  Let  us 
cut  a  prism,  or  small  column  of  ice  with  <be 
planes  of  freezing  lunning  across  it  at  right 
angles  ;  we  place  that  prism  between  two 
slabs  of  wood,  and  bring  carefully  to  bear 
upon  it  the  squeezing  force  of  a  'small  hy- 
draulic press. 

430.  It  is  well  to  converge  by  means  of  a 
concave  mirror  a  good  light  upon  the  ice, 
and  to  view  it  through  a  magnifying  lens. 
You  already  see  Ihe  result.     Hazy  surfaces 
are  formed  in  the  very    body    ct    the  ice, 
which  gradually  expand  as  ttie  pressure  is 
slowly  uugmenltu.       Heie    an-d    there  you 
notice  something  resembling  crystallization  ; 
fern-shaped  figures    run    with  consideiable 
rapidity  through  the  ice,  and  when  you  look 
carefully  at  their  points  and  edges  you  find 
them  in  visible  motion.      These  hazy  sur- 
faces are  spaces  of    liquefaction,    and    the 
motion  you  see  is  that  of  the  ice  falling  to 
water  under  the  pressure.      That  water  is 
colder  than  the  ice  was  before  the  pressure 
was  applied,  and  if  the  pressure  be  relieved 
not  only  does  the  liquefaction  cease,  but  the 
water  re-freezes.     The  cold  produced  by  its 
liquefaction  under  pressure    is  sufficient  to 
re-congeal  it  when  the  pressure  is  removed. 

431.  If  instead  of  diffusing  the  pressure 
over  surfaces  of  consideiable  extent,  we  con- 
centi ate  it  on  a  small  surface,  the  liquefac- 
tion will  of  course  be  more  rapid,  and  this  is 
what  Mr.  Bottomley  has  recently  done  in  an 
experiment  of  singular  beauty  and  interest. 
Let  us  support  on  blocks  of  wood  the  two 
ends  of  a  bar  of  ice  10  inches  long,  4  inches 
deep,  and  3  wide,  and  let  us  loop  over  its 
middle  a  copper  wire  one  twentieth,  or  even 
one  tenth,  of  an  inch  in  thickness.     Con- 
necting the  two  ends  of  the  wire  together, 
and  suspending  from  it  a  weight  of  12  or  14 
pounds,  the  whole  pressure  of  this  weight  is 
concentrated  on  the  ice  which  supports  the 
wire.     What  is  the  consequence  ?    The  ice 
underneath  the  wire  liquefies  ;  .the  water  of 
liquefaction  escapes  round  the  wire,  but  the 
moment  it  is  relieved  from  the  pressure  it 
freezes,  and  round  about  the  wire,  even  be- 
fore it  has  entered  the  ice,  you  have  a  frozen 
casing.     The  wire  continues  to  sink  in  the 
ico  ;  the  water  incessantly  escapes,  freezing 
as  it  does  so  behind  the  wire.     In  half  an 
hour  the  weight  falls  ;   the  wire  has  gone 
clean  through  the  ice.     You  can  plainly  see 
where  it  has  passed,  but  the  two  severed 
pieces  of  ice  are  so  firmly  frozen  together  that 
they  will  break  elsewhere  as  soon  as  along 
the  surface  of  regelation. 

432.  Another  beautiful  experiment  bear, 
ing  upon  this  point  has  recently  been  made 
by  M.  Boussingault.     He  filled  a  hollow  steel 
cylinder  with  water  and  chilled  it.     In  pass- 
ing  to  ice,  water,  as  you  know,  expands 
(§  45) ;  in  fact,  room  for  expansion  is  a  nec- 
essary condition  of  solidification.    But  in  the 
present  case  the  strong  steel  resisted  the  ex- 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


139 


pansion,  the  water  in  consequence  remaining 
liquid  at  a  temperature  of  more  than  30° 
Fahr.  below  the  ordinary  freezing-point.  A 
bullet  within  the  cylinder  rattle"!  about  at 
this  temperature  si  owing  that  the  water  was 
sti!.1  liquid.  On  opening  the  tap  the  liquid, 
relieved  of  the  pressure,  was  instantly  con- 
verted into  ice. 

433.  It  is  only  substances  which  expand  on 
solidifying  that  behave  in  this  manner.     The 
metal  bismuth,  as  we  know,  is  an  example 
similar  to  water  ;  while  lead,  wax,  or  sulphur, 
all  of  which  contract  on  solidifying,   have 
their  point  of  fusion  heightened  by  pressure. 

434.  And  now  you  are  piepared  to  under- 
stand Professor  James  Thomson's  theory  of 
regelation.      When  two   pieces   of   ice   are 
pressed  together,  liquefaction,  he  contends, 
results.     The  water  spreads  out  around  the 
points  of  pressure,  and  when  released    re- 
freezes,  thus  forming  a  kind  of  cement  be- 
tween the  pieces  of  ice. 

§  63.  FARADAY'S  VIEW  OP  REGELATION. 

435.  Faraday's  view  of  regelation   is  not 
so  easily  expressed,  still  I  will  try  to  give, 
you  some  notion  of  it,  dealing  in  the  iirst 
place  with  admitted  facts.     Water,  even  in 
open  vessels,  may  be  lowered  many  degrees 
below  its  freezing  temperature,  and  still  re- 
main liquid  ;  it.  may  also  be  raised  to  a  tem- 
perature far   higher  than  its  boiling-point, 
and  still  resist  boiling.     This  is  due  to  the 
mutual  cohesion  of  the  water  particles,  which 
resists  the  change  of  the  liquid  either  into 
the  solid  or  the  vaporous  condition. 

436.  But  if  into  the  over-chilled  water  you 
throw  a  particle  of  ice.  the  cohesion  is  rup- 
tured, and  congelation  immediately  sets  in- 
And  if  into  the  superheated  water  you  intro- 
duce a  bubble  of  air  or  of  st^am,  cohesion  is 
iikwise  ruptured,  and  ebullition  immediate- 
ly commences. 

437.  Faraday  concluded  that  in  the  interior 
of  any  body,  whether  solid  or  liquid,  where 
every  particle  is  grasped,  so  to  speak,  by  the 
surrounding    particles,    and  grasps  them  in 
turn,  the  bond  of  cohesion  is  so  strong  as  to 
require  a  higher  temperature  to  change  the 
stat?  of  aggregation  than  is  necessary  at  the 
surface.     At  the  surface  of  a  piece  of  ice,  for 
example,  the  molecules  arc  free  on  one  side 
from  the  control  of  other  molecules  ;    and 
they  therefore  yield  to  heat  moie  reaclily  than 
in  the  interior.     The  bubble  of  air  or  steam 
in  overheated  water  also  frees  the  molecules 
on  one  side  ;  hence  the  ebullition  consequent 
upon  its  introduction.     Practically  speaking, 
then,  the  point  of  liquefaction  of  the  interior 
ice  is  higher  than  that  of  the  superficial  ice. 
Faraday  also  refers  to  the  special  solidifying 
power  which  bodies  exert   upon  their  own 
molecules.     Camphor  in  a  glass  bottle  fills 
the  bottle  with  an  atmosphere  of  camphor. 
In  such  an  atmosphere  large  crystals  of  the 
substance  may  grow  by  the  incessant  deposi- 
tion of  camphor  molecules  upon  camphor,  at 
a  temperature  too    high    to    permit  of  the 
slightest  deposit  upon  the  adjacent  glass.     A 
similar  remark  applies  to  sulphur,  phospho- 


rus, and  the  metals  in  a  state  of  fusion. 
They  are  deposited  upon  solid  portions  of 
their  own  substance  at  temperatures  not  low 
enough  to  cause  them  to  solidify  against 
Other  subst3nces. 

488.  Water  furnishes  an  eminent  example 
of  this  special  solidifying  power.  It  may  be 
cooled  ten  degrees  and  more  below  its  freez- 
ing-point without  freezing.  But  this  is  not 
possible  if  the  smallest  fragment  of  ice  be 
floating  in  the  water.  It  then  freezes  accu- 
rately at  32°  Fahr.,  depositing  itself,  how- 
ever, not  upon  th«  sides  of  the  containing 
vessel,  but  upon  the  ice.  Faraday  observed 
in  a  freezing  apparatus  thin  crystals  of  ice 
growing  in  ice-cold  water  to  a  length  of  six, 
eight,  or  ten  inches,  at  a  temperature  incom- 
petent to  produce  their  deposition  upon  the 
sides  of  the  containing  vessel. 

430.  And  now  we  are  prepared  for  Fara- 
day's view  of  regelation.  When  the  surfaces 
of  two  pieces  of  ice,  covered  with  a  film  of 
the  water  of  liquefaction,  are  brought  to- 
gether, the  covering  film  is  transferred  from 
the  surface  to  the  centre  of  the  ice,  where 
the  point  of  liquefaction,  as  before  shown, 
is  higher  than  at  the  surface.  The  special 
solidifying  power  of  ice  upon  water  is  now 
brought  into  play  on  both  sides  of  the  film. 
Under  these  circumstances,  Faraday  held 
that  the  film  would  congeal,  and  freeze  the 
two  surfaces  together. 

440.  The  lowering  of  the  freezing-point 
by  pressure  amounts  to  no  more  than  one 
seventieth  of  a  degree  Fahrenheit  for  a  whole 
atmosphere.     Considering   the   infinitesimal 
fraction  of  this  pressure  which  is  brought 
into  play  in  some  cases  of  regelation,  Fara- 
day thought  its  effect  insensible.     He  sus- 
pended pieces  of  ice,  and  brought  them  into 
contact  without  sensible  pressure,  still  they 
froze  together.     Professor  James  Thomson, 
however,  considered  that  even  the  capillary 
attraction  exerted  between  two  such  masses 
would  be  sufficient  to  produce   regelatiou. 
You  may  make  the  following  experiments, 
in  further  illustration  of  this  subject : 

441.  Place  a  small  piece  of  ice  on  water, 
and  press  it  underneath  the  surface  by  a 
second  piece.      The  submerged  piece  may 
be  so  small  as  to  render  the  pressure  infini- 
tesimal ;  still  it  will  freeze  to  the  under  sur- 
face of  the  superior  piece. 

443.  Place  two  pieces  of  ice  in  a  basin  of 
warm  water,  and  allow  them  to  come  to- 
gether ;  they  freeze  together  when  they 
touch.  The  parts  surrounding  the  place  of 
contact  melt  away,  but  the  pieces  continue 
for  a  time  united  by  a  narrow  bridge  of  ice. 
The  bridge  finally  melts,  and  the  pieces  for  a 
moment  are  separated.  But  capillary  attrac- 
tion immediately  draws  them  together,  and 
regelation  sets  in  once  more.  A  new  bridgo 
is  formed,  which  in  its  turn  is  dissolved,  the 
separated  pieces  again  closing  up.  A  kind 
of  pulsation  is  thus  established  between  the 
two  pieces  of  ice.  They  touch,  they  freeze, 
a  bridge  is  formed  and  melted  ;  and  thus  the 
rhythmic  action  continues  until  the  ice  di& 
appears. 


130 


THE  FORMS  OI   WAT.LR 


443.  According  to  Professor  James  Thom- 
son's theory,  pressure  is  necessary  to  lique- 
fy the  ice.     The  heat  necessary  for  liquefac- 
tion must  be  drawn  from  the  ice  itself,  and 
the  cold  water  must  escape  from  the  pressure 
to  be  re-frozen.     Now  in  the  foregoing  ex- 
periments the  cold  water,  instead  of  being 
allowed  to  freeze,  issues  into  the  warm  water, 
still  the  floatiEg  fragments  regelate  in  a  mo- 
ment.     The  touching  surfaces  may,  more- 
over, be  convex  ;  they  may  be  reduced  prac- 
tically to  points,  clasped  all  round    by   the 
warm  water,  which  indeed  rapidly  dissolves 
them  as  they  approach  each  other  ;  still  they 
freeze  immediately  when  they  touch. 

444.  You  may  learn  from  this  discussion 
that  in  scientific  matters,  as  in  all  others, 
there  is  room   for   differences   of   opinion. 
The  frame  of  mind  to  be  cultivated  here  is  a 
suspension  of  judgment  as  long  as  the  mean- 
ing remains  in  doubt.     It  may  be  that  Fara- 
day's action  and  Thomson's  action  come  both 
into  play.     I  cannot  do  better  than  finish 
these  remarks  by  quoting  Faraday's  own  con- 
cluding words,  which  show  how  in  his  mind 
scientific  conviction  dwelt  apart  from  dog- 
matism :    "No  doubt,"  he  says,  "nice  ex- 
perknents  will  enable  us  hereafter  to  criticise 
such  results  as  these,  and  separating  the  true 
from  the  untrue  will  establish  the  correct 
theory  of  regelation. ' ' 

§  Gi.  THE  BLUE  VEINS  OF  GLACIERS. 

445.  We  now  approach  the  end,  one  im- 
portant question  only  remaining  to  be  dis- 
cussed.    Hitherto  we  have  kept  it  back,  for 
a  wide  acquaintance  with  the  glaciers  was 
necessary  to  its  solution.     We  had  also  to 
make  ourselves  familiar  by  actual  experiment 
with  the  power  of  ice,  softened  by  thaw,  to 
yield  to  pressure,  and  to  liquefy  under  such 
pressure. 

446.  Snow  is  white.     But  if  you  examine 
Its  individual  particles  you  would  call  them 
transparent,     not    white.       The    whiteness 
arises  from  the  mixture  of  the  ice  particles 
with  small  spaces  of  air.     In  the  case  of  all 
transparent  bodies,  whiteness    results   from 
such  a  mixture.     The  clearest  glass  or  crys- 
tal when  crushed  becomes  a  white  powder. 
The  foam  of  champagne  is  wrhitc   through 
the  intimate   admixture    of   a    transparent 
liquid  with  transparent  carbonic  acid  gas. 
The  whitest  paper,  moreover,  is  composed  of 
fibres  which  are  individually  transparent. 

447.  It  is  not,  however,  the  air  or  the  gas, 
but  the  optical  severance  of  the  particles,  giv- 
ing rise  to  a  multitude  of  reflections  of  the 
white  solar  light  at  their  surfaces,  that  pro- 
duces the  whiteness. 

£48.  The  whiteness  of  the  surface  of  a  clean 
glacier  (112),  and  of  the  icebergs  of  the 
Margelm  See  (357),  has  been  already  referred 
lo  a  similar  cause.  The  surface  is  broken 
into  innumerable  fissures  by  the  solar  heat, 
the  reflection  of  solar  light  from  the  sides  of 
the  little  fissures  producing  the  observed  ap- 
pparance. 

449.  In  like  manner  if  you  freeze  water  in 
a  test-tube  by  plunging  it  into  a  freezing 


mixture,  the  ice  produced  is  white.  For  the 
most  part  also  the  ice  formed  in  freezing  ma- 
chines is  white.  Examine  such  ice,  and  you 
will  find  it  filled  with  smaU  air-bubbles. 
When  the  freezing  is  extremely  slow  the 
crystallizing  force  pushes  the  air  effectually 
aside,  and  the  resulting  ice  is  transparent  • 
when  the  freezing  is  rapid,  the  air  is  entan- 
gled before  it  can  escape,  and  the  ice  is 
translucent.  But  even  in  the  case  of  quick 
freezing  Mr.  Faraday  obtained  transparent 
ice  by  skilfully  removing  the  air-bubbles,  as 
fast  as  they  appealed,  with  a  feather. 

45U.  In  the  case  of  lake  ice  the  freezing  is 
not  uniform,  but  intermittent.  It  is  some- 
times slow,  sometimes  rapid.  When  slow 
the  air  dissolved  in  the  water  is  effectually 
squeezed  out  and  forms  a  layer  of  bubbles  on 
the  under  surface  of  the  ice!  An  act  of  sud 
den  freezing  entangles  this  air,  and  hence  we 
find  lake  ice  usually  composed  of  layers  alter- 
nately clear,  and  filled  with  bubbles,  buci? 
layers  render  it  easy  to  detect  the  planes  of 
freezing  in  lake  ice. 

451.  And  now  for  the  bearing  of  these  facts. 
Under  the  fall  of  the  Geant,  at  the  base  of 
the  Talefre  cascade,  and  lower  down  the  Mer 
de  Glace  ;  in  the  higher  regions  of  the  Grin- 
delwald,  the  Aar,  the  Aletsch  and  the  Gor- 
ner  glaciers,   the  ice  does  not  possess  the 
transparency  which  it  exhibits  near  the  ends 
of    the  glaciers.     It   is  white,    or   whitish. 
Why  ?    Examination   shows  it  to  be  filled 
with  small  air-bubbles  ;    and    these,   as  we 
now  learn,  are  the  cause  of  its  whiteness. 

452.  They  are  the  residue  of  the  air  orig- 
nally  entangled  in  the  snow,  and  connected, 
as  before  stated,  with  the  whiteness  of  the 
snow.     During  the  descent  of  the  glacier, 
the  bubbles  are  gradually  expelled  'by  the 
enormous  pressures  brought  to  bear  upon  the 
ice.     Not  on4y  is  the  expulsion  caused  by  the 
mechanical  yielding  of  the  soft  thawing  ice, 
but  the  liquefaction    of    the  substance    at 
places  of   violent  pressure,    opening,    as  it 
does,  fissures  for  the  escape  of  the  air,  must 
play  an  important  part  in  the  consolidation 
of  the  glacier. 

453.  The  expulsion  of  the  bubbles  is,  how- 
ever, not  uniform  ;   for  neither  ice  nor  any 
other  substance  offers  an  absolutely  uniform 
resistance  to  pressure.     At  the  base  of  every 
cascade  that  we  have   visited,  and  on  the 
walls  of  the  crevasses  there  formed,  we  have 
noticed    innumerable    blue    streaks    drawn 
through  the  white  translucent  ice,  and  giv- 
ing the  whole  mass  the  appearance  of  lamina, 
tion.     These  blue  veins  turned  out  upon  ex- 
amination to  be  spaces  from  which  the  air- 
bubbles  had  been  almost  wholly   expelled, 
translucency  being  thus  converted  into  trans 
parency. 

454.  This  is  the  veined  or  ribboned  structure 
of  glaciers,   regarding  the  origin  of  which 
diverse  opinions  are  now  entertained. 

455.  It  is  now  our  duty  to  take  up  the 
problem,  and  to  solve  it  if  we  can.     On  the 
neves  of  the  Col  du  Geant,  and  other  gla- 
ciers,   we    have    found    great    cracks,  and 
faults,  and  Bergschrunds,  exposing  deep  sec- 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIEK8. 


231 


ttons  of  the  neve  ;  aiirl  on  these  sections  we 
have  found  marked  the  edges  of  half-consol- 
idated strata  evidently  produced  by  succes- 
sive falls  of  snow.  The  neve  is  stratified 
because  its  supply  of  material  from  the  at- 
mosphere is  intermittent,  and  when  we  first 
observed  the  blue  veins  we  were  disposed  to 
regard  them  as  due  to  this  stratification. 

450.  But  observation  and  reflection  soon 
dispelled  this  notion.  Indeed,  it  could  hard- 
ly stand  in  the  presence  of  the  single  fact 
that  at  the  ba^es  of  the  ice  falls  the  veins  are 
always  verttctd,  or  neaily  so  We  saw  no 
way  of  explaining  how  the  hoiizontal  strata 
of  the  lien;  could  be  eo  tilted  up  at  the  base 
of  the  fall  as  to  be  set  on  edge.  Nor  is  the 
aspect  of  the  veins  that  of  stratification. 

457.  On  the  central  portions  of  the  cas- 
cades, moreover,  there  are  no  signs  of  the 
veins.  At  the  bases  they  first  appear,  reach- 
ing in  .  each  case  tliei'*  maximum  develop- 
ment a  little  below  the  ba.se.  As  you  and  I 
stood  upon  the  heights  above  the  Zasenberg 
And  scrutinized  the  cascade  of  the  Strahlcck 


tranch  of  the  Grmaelwald  glacier,  we  could 
jjpt  doubt  that  the  base  of  the  fall  was  the 
birthplace  of  the  veins.  "We  called  this  por- 
tion of  the  glacier  a  "  Structure  Mill, "  inti- 


mating that  here,  and  not  on  l?:?,  i»eve,  tne 
veined  structure  was  manufactured. 

458.  This,  however,  is,  at  bottom,  the  lan- 
guage of  strong  opinion  merely,  not  tha-t  of 
demonstration  ;  and  in  science  opinion  ought 
to  content  us  only  so  long  as  positive  proof 
is  unattainable.  "  The  love  of  repose  must 
not    prevent    us  from  seeking    this  proof. 
There  is  no  sterner  conscience  than  the  sci- 
entific conscience,  and  it  demands,  in  every 
possible  case,   the    substitution    for  private 
conviction  of  demonstration  which  s^all  1m 
conclusive  to  all. 

459.  Let  us,  for  example,  be  shown  r,  case 
in  which  the  stratification  of  the  neve  is  pro- 
longed into  the  glacier  ;  let  us  see  the  planes 
of  bedding  and  the  planes  of  lamination  ex- 
isting side  by  side,  and  still  indubitably  dis- 
tinct.    Such  an  observation  would  effectual- 
ly exclude  stratification  from  the  problem  of 
the  veined  -structure,  and   through  the  re- 
moval of  this  tempting  source  of.  error  we 
should  be  icnd^ea  more  free  to  pursue  the 
truth. 

,  460.  We  sought  for  this  conclusive  test 
upon  the  Mer  de  Glace,  but  did  nut  find  it. 
VVe  sought  it  on  the  Grindehvald,  and  the 
Aar  glaciers,  with  an  equal  want  of  suc- 
cess. On  Hie  Aletsch  glacier,  for  the  first 
time,  we  observed  the  apparent  coexistence 
of  bedding  and  structure,  the  one  cutting  the 
other  upon  the  walls  of  the  same  crevasse. 
Still  the  case  was  not  sufficiently  pronounced 
to  produce  entire  conviction,  and  we  visited 
the  Goiner  glacier  with  the  view  of  follow- 
ing up  our  quest. 

461.  Here  day  after  day  added  to  the  con- 
viction that  the  bedding  and  the  structure 
were  two  different  things.     Still  day  after 
day  passed  without  revealing  to  us  the  final 
proof.     Surely  we  have  not  let  our  own  ease 
stand  in  the  way  of  its  attainment,  and  if  we 
retire  baffled  we  shall  do  so  with  the  con- 
sciousness of  having  done  our  best.     Yon- 
der, however,  at  the  base  of  the  Matterhorn, 
is  the  Furgge  glacier  that  we  have  not  }-et 
explored.     Upon  it  our  final  attempt  must  bo 
made. 

462.  We  get  upon  the  glacier  near  its  end, 
and  ascend  it.     We  are  soon  fronted  by  a 
barrier  composed  of  three  successive  walls  of 
neve,  th3  one  rising  above  the  other,  and 
each  ictreating  behind  the  other.     The  bot- 
tom of  each  wall  is  separated  from  the  top  of 
the  succeeding  one   by  a  ledge,  on  which 
threatening  masses  of  broken  neve  now  rest. 
We  stand  amid  blocks  and  rubbish  which 
have  been  evidently  discharged  from  these 
ledges,  on  which  other  masses,  ready  appar- 
ently to  tumble,  are  now  poised. 

463.  On  the  vertical  walls  of  this  barrier 
we  see,  marked  with  the  utmost  plainness, 
the  horizontal  lines  of    stratification,  while 
something  exceedingly  like  the  veined  struc- 
ture appears  to  cross  the  lines  of  bedding  at 
nearly  a  right  angle.     The  vertical  surface 
is,  howe/er, weathered,  and  the  lines  of  struc- 
ture, if   aiey  be  such,  are  indistinct.     The 
problem  now  is  to  remove  the  surface,  and 
expose  the  ice  underneath.     It  is  one  of  the 


132 


THE  FORMS  OF  WATER 


many  cases  that  have  come  before  us,  where 
the  value  of  au  observation  Is  to  be  balanced 
against  the  danger  which  it  involves. 

464.  We  do  nothing  rashly  ;  but  scanning 
the  ledges  and  selecting  a  point  of  attack, 
we  conclude  that  the  danger  is  not  too  great 
to  be  incurred.     We  advance  to  the  wall, 
remove  the  surface,  and  are  rewarded  by  the 
discovery   underneath  it   of    the    true  blue 
veins.     They,  moreover,   are  vertical,  while 
the   bedding  is  horizontal .     Bruce,  as  you 
know,  was  defeated  in  many  a  battle,  but  he 
persisted  and  won  at  last.     Hero,  upon  the 
Furgge  glacier,  you  also  have  fought  and 
won  your  little  Bannockburn. 

465.  But  let  us  not  use  the  language  of 
victory  too  soon.     The  stratification  theory 
has  been  removed  out  of  the  field  of  expla- 
nation, but  nothing  has  as  yet  been  offered 
in  its  place. 

§  05.  RELATION  OF  STRUCTURE  TO 
PRESSURE. 

460.  This  veined  structure  was  first  de- 
scribed by  the  distinguished  Swiss  naturalist, 
Guyot,  now  a  resident  in  the  United  States. 
From  *he  Grimsel  Pass  I  have  already 
pointed  out  to  you  the  Giles  glacier  over- 
spreading the  mountains  at  Ihe  opposite  side 
of  the  valley  of  the  Rhone.  It  was  on  this 
glacier  that  M.  Guyot  made  his  observation. 

467.  "I  saw,"  he  said,  "under  my  feet 
the  surface  of  Ihe  entire  glacier  covered  with 
regular  furrows,   from  one  to  two  inches 
wide,  hollowed  out  in  a  half-snowy  mass, 
and  separated  by  protruding  plates  of  harder 
and  more  transparent  ice.     It  was  evident 
that  the  glacier  here  was  composed  of  two 
kinds  of  ice,  one  that  of  the  furrows,  snowy 
and  more  easily  melted  ;  the  other  of  the 
plates,  more  perfect,  crystalline,  glassy,  and 
resistant  ;    and  that  the  unequal  resistance 
which  the  two  kinds  of  ice  presented  to  the 
atmosphere  was  the  cause  of  the  ridges. 

468.  "  After  having  followed  them  for  sev- 
eral hundred  yards,   I  reached  a  crevasse 
twenty  or  thirty  feet  wide,  which,  as  it  cut 
the  plates  and  furrows  at  right  angles,  ex- 
posed the  interior  of  the  glacier  to  a  depth  of 
thirty  or  forty  feet,  and  gave  a  beautiful 
transverse  section  of  the  structure.   As  far  as 
my  eyes  could  reach,  I  saw  the  mass  of  the 
glacier  composed  of  layers  of  snowy  ice,  each 
two  of  which  were  separated  by  one  of  the 
hard  plates  of  which  1  have  spoken,  the 
whole  forming  a  regularly  laminated  mass, 
which  resembled  certain  calcareous  slates." 

469.  I  have  not  failed  to  point  out  to  you 
upon  all  the  glaciers  that  we  have  visited  the 
little  superficial  furrows  here  described  ;  and 
you  have,  moreover,  noticed  that  in  the  fur- 
rows mainly  is  lodged  the  finer  dirt  which  is 
scattered  over  the  glacier.    They  suggest  the 
passage  of  a  rake  over  the  ice.     And  when- 
ever these  furrows  were  interrupted  by  a  cre- 
vasse, the  veined   structure   invariably    re- 
vealed itself  upon  the  walls  of  the  fissure. 
The  surface  grooving  is  indeed  an  infallible 
indication  of  the  interior  lamination  of  Has 
ice. 


IN  CLOUDS  AND  RIVERS,  ICE  AND  GLACIERS. 


470.  We  have  tracked  the  structure  through 
the  various  parts  of  the  glaciers  at  which  its 
appearance  was  most  distinct ;  and  we  have 
paid  particular  attention  to  the  condition  of 
the  ice  at  these  places.     The  very  fact  of  its 
cutting  the  crevasses  at  right  angles  is  sig- 
nificant.    We  know  the  mechanical  origin  of 
the  crevasses  ;  that  they  are  cracks  formed 
at  right  angles  to  lines  of  tension.     But  since 
the  crevasses  are  also  perpendicular  to  the 
planes  of  structure,   these  planes  must  be 
parallel  to  the  lines  of  tension. 

471.  On    the    glaciers,   however,    tension 
rarely  occurs  alone.     At  the  sides  of  the  gla- 
cier, for  example,  where  marginal  crevasses 
are  formed,  the  tension  is  always  accom- 
panied by  pressure  ;  the  one  force  acting  at 
right  angles  to  the  other.     Here,  therefore, 
the  veined  structure,  which  is  parallel  to  the 
lines  of  tension,  is  perpendicular  to  the  lines  of 
pressure. 

472.  That  this  is  so  will  be  evident  to  you 
in  a  moment.     Let  the  adjacent  figure  rep- 


FIG.  1!). 

resent  the  channel  of  the  glacier  moving  in 
the  direction  of  the  arrow.  Suppose  three 
circles  to  be  marked  upon  the  ice.  one  at  the 
centre  and  the  two  others  at  the  sides.  In  a 
glacier  of  uniform  inclination  all  these  circles 
would  move  downward,  the  central  one  only 
remaining  a  circle.  By  the  retardation  of 
the  sides  the  marginal  circles  would  be  drawn 
out  to  ovals.  The  two  circles  would  be 
elongated  in  one  direction,  and  compressed  in 
another^  Across  the  long  diameter,  which 
is  the  direction  of  strain,  we  have  the  mar- 
ginal crevasses  :  across  the  short  diameter 
m  n,  which  is  the  direction  of  pressure,  we 
have  the  marginal  veined  structure. 

473.  This  association  of  pressure  and  struc- 
ture is  invariable.     At  the  bases  of  the  cas- 
cades, where  the  inclination  of  the  bed  of  the 
glacier  suddenly  changes,   the  pressure  in 
many  cases  suffices  not  only  to  close  the  cre- 
vasses but  to  violently  squeezs  the  ice.     At 
such  places  the  structure  always  appears, 
sweeping  quite  across  the   glacier.     When 
two    branch    glaciers    unite,    their    mutual 
thrusi   i  i  tensities  the  pie-existing  marginal 
utructure  of  the  branches,  and  develops  new 
planes  of  lamination.     Under  the  medial  mo- 
raines, therefore,  we  have  usually  a  good  de- 
velopment of  the  structure.     It  is  finely  dis- 
played, for  example,  under  the  great  medial 
moraine  of  the  glacier  of  the  Aaf. 

474.  Upon  this  glacier,  indeed,  the  blue 
veins   were    observed    independently    three 
years  after  M.    Guyot  had   first    described 
them.      I    say  independently,    because    M. 
Guyot's  description,  though  written  in  1838, 
remained  unprinted,  and  was  unknown  in 


1841  to  the  observers  on  the  Aar.  These 
were  M.  Agassi z  and  Professor  Forbes.  To 
the  question  of  structure  Professor  Forbes 
subsequently  devoted  much  attention,  and  it 
was  mainly  his  observations  and  reasoning** 
that  gave  it  the  important  position  now  as- 
signed to  it  in  the  phenomena  of  glaciers. 

475.  Thus  without   quitting  the  glaciers 
themselves,  we  establish  the  connection  bo 
tween  pressure  and  structure.     Is  there  any- 
thing in  our  previous  scientific  experience 
with  which  these  facts  may  be  connected  ? 
The  new  knowledge  of  nature  must  always 
strike  its  roots  into  the  old,  and  spring  from 
it  as  an  organic  growth. 

§    GG.    SLATE    CLEAVAGE     AND    GLACIER 
LAMINATION. 

476.  M.  Guyot  threw  out  an  exceedingly- 
sagacious    hint,    when     he    compared     the 
veined  structure  to  the  cleavage   of    slate 
rocks.     We  must  learn  something  of  this 
cleavage,  for  it  really  furnishes  the  key  to 
the  problem  which  now  occupies  us.     Let  us 
go  then  to  the  quarries  of  Bangor  or  Cum- 
berland, and  observe  the  quarrymen  in  their 
sheds  splitting    the   rocks.     With  a    sharp 
point  struck  skilfully  into  the  edge  of  the 
slate,  they  cause  it  to  divide  into  thin  plates, 
fit  for  roofing  or  ciphering,  as  the  case  may 
be.     The  surfaces    along    which    the   rock 
cleaves  are  called  its  planes  of  cleavage. 

477.  All  through  the  quarry  you  notice  the 
direction  of  these  planes  to  be  perfectly  con- 
stant.    How  is  this  laminated  structure  to  be 
accounted  for  ? 

478.  You  might  be  disposed  to  consider 
that  cleavage  is  a  case  of  stratification  or 
bedding  ;  for  it  is  true  that  in  various  parts 
of  England  there  are  rocks  which  can  be 
cloven  into  thin  flags  along  the  planes  of 
bedding.     But  when  we  examine  these  slate 
rocks  we  verify  the  observation,  first  I  be- 
lieve made  by   the  eminent  and  venerable 
Professor  Sedgwick,  that  the  planes  of  bed- 
ding usually  run  across  the  planes  of  cleav- 
age^ 

479.  We  have  here,  as  you  observe,  a  case 
exactly  similar  to  that  of  glacier  lamination, 
which  we  were  at  first  disposed  to  icgard  as 
due  to  stratification.     We  afterward,  how- 
ever, found  planes  of  lamination  crossing  the 
layers  of  the  neve,  exactly  as  the  planes  of 
cleavage  cross  the  beds  of  slate  rocks. 

480.  But    the    analogy    extends    further. 
Slate  cleavage  continued  to  be  a  puzzio  to 
geologists  lill  the  late  Mr.    Daniel  Sharpe 
made  the  discovery  that  shells  and  other  fos- 
sils and  bodies  found  in  slate  rocks  are  inva- 
riably flattened  out  in  the  planes  of  cleavage. 

481.  Turn  into  any  well-arranged  museum 
— for  example,  into  the  School  of  Mines  in 
Jermyn   Street,   and    observe  the  evidence 
there   collected.     Look   particularly  to    the 
fossil  trilobites  taken  from   the  s4ate  rock. 
They  are  in  some  cases  squeezed  to  one  third 
of    their    primitive    thickness.     Nurner.ju~ 
other  specimens  show  in  the  most  striking 
manner  the  flattening  out  of  shells. 

482.  To  the  evidence    adduced    by   Mr. 


ISi 


THE  FORMS  OF  WATER 


Sharpe,  Mr.  Sorby  added  other  powerful  evi- 
dence, founded  upon  the  microscopic  exam- 
ination of  slate  rock.  Taking  both  into  ac- 
count, the  conclusion  is  irresistible  that  such 
rocks  have  suffered  enormous  pressure  at 
right  angles  to  the  planes  of  cleavage,  exactly 
as  the  glacier  has  demonstrably  suffered 
great  pressure  at  right  angles  to  its  planes  of 
lamination. 

488.  The  association  of  pressure  and  cleav- 
age is  thus  demonstrated  ;  but  the  question 
arises,  Do  they  stand  to  each  other  in  the  re- 
lation of  cause  and  effect  ?  The  only  way  of 
replying  to  this  question  is  to  combine  artifi- 
cially the  conditions  of  nature,  and  see 
whether  we  cannot  produce  her  result*. 

484.  The  substance  of  slate  rocks  was  once 
a  plastic  mud,  in  which  fossils  were  imbed- 
ded.    Let  us  imitate  the  action  of  pressure 
upon  such  mud  by  employing,  instead  of  it, 
softened  while  wax.     Placing  a  ball  of  the 
wax  between  t\\o  glass  plates,  wetted  to  pre- 
vent it  from  slicking,  we  apply  pressure  and 
flatten  out  the  wax. 

485.  The  flattened  mass  is  sit  first  too  soft 
to  cleave  sharply  ;  but  you  can  see,  by  tear- 
ing, that  it  is  laminated.     Let  us  chill  it  with 
ice.     We  find  aflerwaid  that  no  slate  rock 
ever  exhibited  so  fine  a  cleavage.     The  lam- 
ina?, it  need  hardly  be  said,  are  perpendicular 
to  the  pressure. 

486.  One  cause  of  this  laminnticn  is  that 
the  wax  is  an  aggregate  of  gtanules  the  sur- 
faces of  which  are  places  of  weak  cohesion  ; 
and  that  by  the  pressure  these  granules  are 
squeezed  flat,  thus  producing  planes  of  weak- 
ness at  right  angles  to  the  pressure. 

487.  But  the  main  cause  of  the  cleavage  I 
take  to  be  the  lateral  sliding  of  the  particles 
of  wax  over  each  other.     Old  attachments 
are  thereby  severed,  which  the  new  ones  fail 
to  make  good.     Thus  the  tangential  sliding 
produces  lamination,  as  the  rails  near  a  sta- 
tion are  caused  to  exfoliate  by  the  gliding  of 
the  wheel. 

488.  Instead  of  wax  we  may  take  the  slate 
itself,  grind  it  to  fine  powder,  add  water,  and 
thus  reproduce  the   piistine  mud.     By  the 
proper  compression  of  such  mud,   in  one 
direction,  the  cleavage  is  restored. 

489.  Call  now  to  mind  the  evidences  we 
have  had  of  the  power  of  thawing  ice  to  yield 
to  pressure.     Recollect  the  shortening  of  the 
Glacier  du  Geant,  and  the  squeezing  of  the 
Glacier  de  Lechaud,  at  Trelaporte.     Such  a 
substance,  slowly  acted  upon  by  pressure, 


will  yield  laterally.  Its  part  ides  wih  slide 
over  each  other,  the  severed  attachments  be- 
ing immediately  made  good  by  legi-lation 
It  will  not  yield  uniformly,  but  along  special 
planes.  It  will  also  liquefy,  not  uniformly, 
but  along  special  surfaces.  Both  the  sliding 
and  the  liquefaction  will  take  place  princi- 
pally at  right  angles  to  the  pressure,  and  gla- 
cier lamination  is  the  result. 

490.  As  long  as  it  is  sound  Ihe  laminated 
glacier  ice  resists  cleavage.     Regelatiou,  as  I 
have  said,   makes  the  severed  "attachments 
good.     But  when  such  ice  is  exposed  to  ike 
weather  the  structure  is  revealed,  and  the  ice 
can  then  be  cloven  into  tablets  a  square  foot, 
or  even  a  square  yard  in  area. 

§  67.  CONCLUSION. 

491.  Here,  my  friend,  our  labors  close.     It 
has  been  a  true  pleasure  to  me  to  have  you 
at  my  side  so  long     In  the  sweat  of  our 
brows  we  have  often  reached  the  heights 
where  our  work  lay,   but  you  have  been 
steadfast  and  industrious  throughout,  using 
in  all  possible  cases  your  own  muscles  in- 
stead of  relying  upon  mine.     Here  and  there 
I  have  stretched  an  arm  and  helped  you  to  a 
ledge,  but  the  work  of  climbing  has  been  al- 
most exclusively  your  own.     It  is  thus  that 
I  should  like  to  teach  you  all  things  ;  show- 
ing you  the  way  to  profitable  exertion,  but 
leaving  the  exertion  to  you — more  anxious  to 
bring  out  your  manliness  in  the  presence  of 
difficulty  than  to  make  your  way  snioclh  by 
toning  difficulties  down. 

492.  Steadfast,    prudent,   without    terror, 
though  not  at  all  times  without  awe,  I  have 
found  you  on  rock  and  ice,- and  you  have 
shown  the  still  rarer  quality  of  steadfastness 
in  intellectual  effort.     As  here  set  forth,  our 
task  seems  plain  enough,  but  you  and  I  know 
how  often  we  have  had  to  wrangle  resolutely 
with  the  facts  to  bring  out  their  meaning. 
The  work,  however,  is  now  done,  and  you 
are  master  of  a  fragment  of  that  sure  an;i 
certain  knowledge  which  is  founded  on  the 
faithful  study  of  nature.     Is  it  not  worth  th« 
price  paid  for  it  ?    Or  rather,  was  not  tho 
paying  of  the  price— the  healthful,  if  some 
times  hard,  exercise  of  mind  and  body,  upon 
alp  and  glacier — a  portion  of  our  delight  ? 

493.  Here  then  we  part.     And  should  we 
liot  meet  again,  the  memory  of  these  days 
will  still  unite  us.      Give  rue  your  hand. 
Good-by. 


LESSONS  IN  ELECTRICITY. 


BY 
JOHN  TYNDALL. 


LESSONS  ON  ELECTRICITY. 


CONTENTS: 


Preface,     -  -  287 

Introduction. 

Historic  Notes, 

The  Art  of  Experiment, 

Materials  for  Experiment, 

Electric  Attractions,  -      290 

Disco  s^ery  of  Conduction  and  Insulation, 

The  Electroscope — Further  Inquiries  on  Conduction  and  Insulation,    - 

Electrics  and  Non-Electrics,        -  ~9~> 

Electric  Repulsions. — Discovery  of  Two  Electricities,       -  296 

Fundamental  Law  of  Electric  Action,  29? 

Electricity  of  the  Rubber. — Double  or  "  Polar  "  Character  of  Electric;  Force,       29'J 

What  is  Electricity  ?  301 

Electric  Induction,  Definition  of  the  Term,       -  303 

Experimental  Researches  on  Electric  Induction,  30:3 

The  Electrophorus,  307 

Action  of  Points  and  Flames.  30S 

The  Electrical  Ma.chine,  -  309 

Further  Experiments  on  the  Action  of  Points. — The  Electric-  Mill  — The  Golden 

Fish. — Lightening  Conductors,  -      Oil 

History  of  the  Ley  den  Jar. — The  Leydeii  Battery,      -  314 

Explanation  of  the  Ley  den  Jar,       -  31.) 

Franklin's  Cascade  Battery,        -  31? 

Novel  Leydeh  Jars  of  the  Simplest  Form, 
Seat  of  Charge  in  the  Leydeii  Jar, 

Ignition  by  Electric  Spark.— Cottrel's  Rubber.— The  Tube  Machine.  320 

Duration  of  the  Electric  Spark, 
Electric  Light  in  Vacim, 
Lichtenberg's  Figures, 

Surface  Compared  with  Mass. — Distribution  of  Electricity  in  Hollow  Conductors, 32(J 
Physiological  Effects  of  Electric  Discharges, 
Atmospheric  Electricity, 
Returning  Stroke, 

The  Leyden  Battery,  Its  Currents,  aiid  some  of  their  Effects, 
Conclusion, 
Appendix. — An  Elementary  Lecture  on  Magnetism, 


LESSONS  IN  ELECTRICITY; 


TO   WHICH  IS  ADDED 


AN  ELEMENTARY  LECTURE  ON  MAGNETISM. 


BY 


JOHN  TYNDA'LL,  D.C.L.,  LL.D.,  F.R.S., 

PROFESSOR  OF  NATURAL  PHILOSOPHY  IN  THE  ROYAL  INSTITUTION  OF  GREAT  BRITAIN. 


WITH  SIXTY  ILLUSTRATIONS. 


PREFACE. 

MORE  than  fifty  years  ago  the  Board 
of  Managers  of  the  Royal  Institution  re- 
solved to  extend  its  usefulness,  as  a  centre 
of  scientific  instruction,  by  giving,  during 
the  Christmas  and  Easter  holidays  of  each 
year,  two  courses  of  Lectures  suited  to 
the  intelligence  of  boys  and  girls. 

On  December  12th,  1825,  a  Commit- 
tee appointed  by  the  Managers  reported 
"  that  they  had  consulted  Mr.  Faraday 
on  the  subject  of  engaging  him  to  take  a 
part  in  the  juvenile  lectures  proposed  to 
be  given  during  the  Christmas  and  Easter 
recesses,  and  they  found  his  occupations 
were  such  that  it  would  be  exceedingly 
inconvenient  for  him  to  engage  in  such 
lectures. ' ' 


Faraday's  holding  aloof  was,  however, 
but  temporary,  for  at  Christmas  1827  wf 
find  him  giving  a  "  Course  of  Six  Ele- 
mentary Lectures  on  Chemistry,  adapted 
to  a  Juvenile  Auditory." 

The  Easter  lectures  were  soon  aban- 
doned, but  f  rcm  the  date  mentioned  to  the 
present  time  the  Christmas  lectures  havn 
been  a  marked  feature  of  the  Royal  In- 
stitution.* 

Last  Christmas  it  fell  to  my  lot  to  give 
one  of  these  courses.  I  had  heard  doubts 
expressed  as  to  the  value  of  science-teach- 
ing in  schools,  and  I  had  heard  objec- 
tions urged  on  the  score  of  the  expensive- 
ness  of  apparatus.  Both  doubts  and 

*  These  brief  historic  references  have  al- 
ready appeared  in  the  preface  to  the  "  Forms 
of  Water." 


ELECTRICITY. 


objections  would,  I  considered,  be  most 
practically  met  by  showing  what  could 
be  done,  in  the  way  of  discipline  and  in- 
struction, by  experimental  lessons  involv- 
ing the  use  of  apparatus  so  simple  and 
inexpensive  as  to  be  within  everybody's 
reach. 

With  some  amplification,  the  substance 
of  .our  Christmas  Lessons  is  given  in  the 
present  little  volume. 

LESSONS  IN  ELECTRICITY. 

§  1.   Introduction. 

MANY  centuries  before  Christ,  it  had 
been  observed  that  yellow  amber  (elck- 
tron\  when  rubbed,  possessed  the  power 
of  attracting  light  bodies. 

Thales,  the  founder  of  the  Ionic  philos- 
ophy (B.C.  580),  imagined  the  amber  to 
be  endowed  with  a  kind  of  life. 

This  is  the  germ  out  of  which  has 
grown  the  science  of  electricity,  a  name 
derived  from  the  substance  in  which  this 
pr.ver  of  attraction  was  first  observed. 

It  will  be  my  aim,  during  six  hours  of 
those  Christinas  holidays,  to  make  you, 
to  some  extent,  acquainted  with  the  his- 
tory, facts,  and  principles  of  this  science, 
and  to  teach  you  how  to  work  at  it. 

The  science  has  two  great  divisions  : 
the  one  called  "  Frictional  Electricity," 
the  other  "Voltaic  Electricity."  For 
the  present,  our  studies  will  be  confined 
to  the  first,  or  older  portion  of  the  sci- 
ence, which  is  called  "  Frictional  Elec- 
tricity," because  in  it  the  electrical 
power  is  obtained  from,  the  rubbing  of 
bodies  together. 

§  2.  Historic  Note*. 

The  attraction  of  light  bodies  by 
rubbed  amber  was  the  sum  of  the  world's 
knowledge  of  electricity  for  more  than 
2000  years.  In  1600  Dr.  Gilbert,  phy- 
sician to  Queen  Elizabeth,  whose  atten- 
tion had  been  previously  directed  with 
great  success  to  magnetism,  vastly  ex- 
panded the  domain  of  electricity.  He 
showed  that  not  only  amber,  but  various 
spars,  gems,  fossils,  stones,  glasses,  and 
rosins,  exhibited,  when  rubbed,  the  same 
power  as  amber. 

Robert  Boyle  (1675)  proved  that  a 
suspended  piece  of  rubbed  amber,  which 


attracted  other  bolie.*  to  itself,  was  in 
turn  attracted  by  a  body  brought  near  it. 
He  also  observed  the  light  of  electricity, 
a  diamond,  with  which  he  experimented, 
being  found  to  emit  light  when  rubbe  I 
in  the  dark. 

Boyle  imagined  that  the  electrified 
body  threw  out  an  invisible,  glutinous 
substance,  which  laid  hold  of  light  bodie*, 
and,  returning  to  the  source  from  which 
it  emanated,  carried  them  along  with  it. 

Otto  von  Guericke,  Burgomaster  of 
Magleburg,  contemporary  of  Boyle,  and 
inventor  of  the  air-pump,  intensified  the 
electric  power  previously  obtained.  He 
devised  what  may  be  called  the  first  elec- 
trical machine,  which  was  a  ball  of 
sulphur,  about  the  size  of  a  child's  head. 
Turned  by  a  handle,  and  rubbed  by  the 
dry  hand,  the  sulphur  sphere  emitted 
light  in  the  dark. 

Von  Guericke  also  noticed,  and  this  is 
important,  that  a  feather,  having  been 
first  attracted  to  his  sulphur  globe,  was 
afterward  repelled,  and  kept  at  a  dis- 
tance from  it,  until,  having  touched 
another  body,  it  was  again  attracted. 
He  heard  the  hissing  of  the  "  electric 
lire,"  and  also  observed  that  an  unelec- 
trified  body,  when  brought  near  his  ex- 
cited sphere,  became  electrical  and  capa- 
ble of  being  attracted. 

The  members  of  the  Academy  del 
Cimento  examined  various  substances 
electrically.  They  proved  smoke  to  be 
attracted,  but  not  flame,  which,  they 
found,  deprived  an  electrified  body  of  its 
power. 

They  also  proved  liquids  to  be  sensible 
to  the  electric  attraction,  showing  that 
when  rubbed  amber  was  held  over  the 
surface  of  a  liquid,  a  little  eminence  was 
formed,  from  which  the  liquid  was  finally 
discharged  against  the  amber. 

Sir  Isaac  Newton,  by  rubbing  a  flat 
glass,  caused  light  bodies  to  jump  be- 
tween it  and  a  table.  He  also  noticed 
the  influence  of  the  rubber  in  electric  ex- 
citation. His  gown,  for  example,  was 
found  to  be  much  more  effective  than  a 
napkin. 

Newton  imagined  that  the  excited  body 
emitted  an  elastic  fluid  which  penetrated 
glass. 

In  the  ^  efforts  of  Thales,  Boyle,  and 
Newton  to  form  a  mental  picture  of  elec- 
tricity we  have  an  illustration  of  the  ten- 


LESSONS  IN  ELECTRICITY. 


289 


dency  of  the  human  mind,  not  to  rest 
satisfied  with  the  facts  of  observation,  but 
to  pass  beyond  the  facts  to  their  invisible 
causes. 

Dr.  Wall  (1708)  experimented  with 
large,  elongated  pieces  of  amber.  lie 
found  wool  to  be  the  best  rubber  of  am- 
ber. "  A  prodigious  number  of  little 
cracklings"  was  produced  by  the  fric- 
tion, every  one  of  them  beinjj  accom- 
panied by  a  flash  of  light.  "  This  light 
and  crackling,"  says  Dr.  Wall,  "  seem 
in  some  degree  to  represent  thunder  and 
lightning."  This  is  the  first  published 
allusion  to  thunder  and  lightning  in  con- 
nection with  electricity. 

Stephen  Gray  (1729)  also  observed  the 
electric  brush,  {-mappings,  and  sparks. 
He  made  the  prophetic  remark  that 
"  though  these  effects  are  at  present  only 
minute,  it  is  probable  that  in  time  theie 
may  be  found  out  a  way  to  collect  a 
greater  quantity  of  the  electric  fire,  and, 
consequently,  to  increase  the  force  of 
that  power  which  by  several  of  those  ex- 
periments, if  we  are  permitted  to  com- 
p'-irc  great  things  with  small,  seems  to  be 
of  the  same  nature  with  that  of  thiindcF 
and  lightning."  This,  you  will  ob- 
serve, is  far  more  definite  than  the  re- 
mark of  Dr.  Wall. 

§  3.    The  Art  of  Experiment. 

We  have  thus  broken  ground  with  a 
few  historic  notes,  intended  to  show  the 
gradual  growth  of  electrical  science.  Our 
next  step  must  be  to  get  some  knowledge 
of  the  facts  referred  to,  and  to  learn  how 
they  may  be  produced  and  extended. 
The  art  of  producing  and  extending  such 
facts,  and  of  inquiring  into  thorn  by  prop- 
er instruments,  is  the  art  of  experiment. 
It  is  an  art  of  extreme  importance,  for 
by  its  means  we  can,  as  it  were,  converse 
with  Nature,  asking  her  questions  and 
receiving  from  her  replies. 

It  was  the  neglect  of  experiment,  and 
of  the  reasoning  based  upon  it,  which 
kept  the  knowledge  of  the  ancient  world 
confined  to  the  single  fact  of  attraction 
by  amber  for  more  than  2000  years. 

Skill  in  the  art  of  experimenting  docs 
not  come  of  itself  ;  it  is  only  to  be  ac- 
quired by  labor.  When  you  first  take  a 
billiard  cue  in  your  hand,  your  strokes 
are  awkward  and  ill-directed.  When 


you  learn  to  dance,  your  first  movements 
are  neither  graceful  nor  pleasant.  By 
practice  alone,  you  learn  to  dance  and 
to  play.  This  also  is  the  only  way  of 
learning  the  art  of  experiment.  You 
must  not,  therefore,  be  daunted  by  your 
clumsiness  at  first  :  you  must  overcome 
it,  and  acquire  skill  in  tho  art  by  repeti- 
tion. 

In  this  way  you  will  come  into  direct 
contact  with  natural  truth — you  will 
think  and  reason  not  on  what  has  been 
said  to  you  in  books,  but  on  \rhat  has 
been  said  to  you  by  Nature.  Thought 
springing  from  this  source  has  a  vitality 
not  derivable  from  mere  book-knowledge. 

§  4.  Materials  for  Experiment. 

At  this  stage  of  our  labors  wo  arc  to 
provide  ourselves  with  the  fallowing 
materials  : 


FIG.  1. 

a.   Some  sticks  of  sealing-wax  ; 

5.  Two  pieces  of  gutta-percha  tubing, 
about  18  inches  long  and  f  of  an  inct 
outside  diameter  ; 

c.  Two  or  three  glass  tubes,  about  18 
inches  long  and  f  of  an  inch  wide,  closed 
at  one  end,  and  not  too  thin,  lest  they 
should  break  in  your  hand  and  cut  it ; 

d.  Two  or  three  pieces  of  clean  flannel, 
capable  of  being  folded  into  pads  of  two 
ar  three  layers,  about  eight  or  ten  inches 
square  ; 


290 


LESSORS  IN  KI7ECTRIOITY. 


€.  A  couple  of  pads,  composed  of 
three  or  four  layers  of  silk,  about  eight 
or  ten  inches  square  ; 

/.  A  board  about  18  inches  square, 
and  a  piece  of  india-rubber  ; 

<7.  Some  very  narrow  silk  ribbon,  n, 
and  a  wire  loop,  w,  like  that  shown  in 
fur.  1,  in  which  sticks  of  sealing-wax, 
tubes  of  gutta-percha,  rods  of  glass,  or  n 
Walking-stick,  may  be  suspended.  I 
cl)ooso  a  narrow  ribbon  because  it  is  con- 
venient to  have  a  suspending  cord  that 
will  neither  twist  nor  untwist  of  itself. 

(I  usually  employ  a  loop  with  the  two 
ends,  which  arc  here  shown  free,  soldered 
together.  The  loop  would  thus  be  un- 
broken. But  you  may  not  be  skilled  in 
the  art  of  soldering,  and  I  therefore 
choose  the  free  loop,  which  is  very  easily 
constructed.  For  the  purpose  of  suspen- 
sion an  arrangement  resembling  a  towel- 
horse,  with  a  single  horizontal  rail,  will 
be  found  convenient). 


FIG.  2. 

h.  A  straw,  1 i',  fig,  2,  delicately  sup- 
ported on  the  point  of  a  sewing  needle  N. 
This  is  inserted  in  a  stick  of  sealing-wax 
A,  attached  below  to  a  little  circular  plate 
of  tin,  the  whole  forming  a  stand.  In 
fig.  3  the  straw  is  shown  on  a  larger 
scale,  and  separate  from  its  needle.  The 
shore  bit  of  straw  in  the  middle,  which 
serves  as  a  cap,  is  stuck  on  by  sealing- 
wax. 

t.  Tke  name  "amalgam"  is  given  to 
a  mixture  of  mercury  with  other  metals. 
Experience  has  shown  that  the  efficacy  of 
a  silk  rubber  is  vastly  increased  when  it 
is  smeared  over  with  an  amalgam  formed 
of  1  part  by  weight  of  tin,  2  of  zinc,  and 
6  «f  mercury.  A  littlo  lard  is  to  be  first 


smeared  on  the  s-ilk,  and  the  amalgam  is 
to  be  applied  to  the  lard.  The  amalgam. 
if  hard,  must  be  pounded  or  biuised.with 
a  pestle  or  a  hammer  until  it  is  soft. 
You  can  purchase  sixpenny- worth  of  it  at 
a  philosophical  instrument  maker's.  It 
is  to  be  added  to  your  materials. 

fc.  I  should  like  to  make  these  pages 
suitable  for  boys  without  much  pocket- 
money,  and,  therefore,  aim  at  economy 
hi  my  list  of  materials.  But  provide  by 
all  means,  if  you  can,  a  fox's  brush,  such 


as   those    usually    employed   in   dusting 
furniture. 

§  5.   Electric  Attractions. 

Place  your  sealing-wax,  gutta-percha 
tubing,  and  flannel  and  silk  rubbers  be- 
fore a  fire,  to  insure  their  dryncss.  Be 
specially  careful  to  make  your  glass 
tubes  and  silk  rubbers  not  only  warm, 
but  hot.  Pass  the  diied  flannel  briskly 
once  or  twice  over  a  stick  of  sealing-wax 
or  over  a  gutta-percha  tube.  A  very 
small  amount  of  friction  will  excite  the 
power  of  attracting  the  suspended  straf 
as  shown  in  fig.  2.  Repeat  the  experi- 
ment several  times  and  cause  the  straw  to 
follow  the  attracting  body  round  and 
round.  Do  the  same  with  a  glass  tube 
rubbed  with  silk. 

I  lay  particular  stress  on  the  heating  of 
the    glass   tube,    because   glass   has   the 


LESSONS  IN  ELECTRICITY. 


231 


power,  wliich  it  exercises,  of  condensing 
upon  its  surface  into  a  liquid  film,  the 
{i  <  I  neons  vapor  of  the  surrounding  air. 
This  film  must  be  removed. 

1  would  also  insist  on  practice,  in  order 
to  render  you  expert.    You  will  therefore 


attract  bran,  scraps  of  paper,  gold  leaf, 
soap  bubbles,  and  other  light  bodies  by 
rubbed  glass,  sealing-wax,  and  gutta- 
perciia.  Faraday  was  fond  of  making 
empty  egg  shells,  hoops  of  paper,  and 
other  light  objects  roll  after  his  excited 
tubes. 

It  is  only  when  the  electric  power  is 
very  weak,  that  you  require  your  deli- 
cately suspended  straw.  With  the  sticks 
of  wax,  tubes,  and  rubbers  here  men- 
tioned, even  heavy  bodies,  when  properly 
suspended,  may  be  attracted.  Place,  for 
instance,  a  common  walking  stick  in  the 
wire  loop  attached  to  the  narrow  ribbon, 
fig.  .1,  ;md  let  it  swing  horizontally.  The 
glass,  rubbed  with  its  silk,  or  the  sealing- 
wax,  or  gutta-percha,  nibbed  with  its 
flannel,  will  pull  the  stick  quite  round. 

Abandon  the  wire  loop  ;  place  an  egg 
in  an  (gg-cup,  and  balance  a  long  lath 
upon  the  egg,  as  shown  in  fig.  4.  The 
lath,  though  it  may  be  almost  a  plank, 


will  obediently  follow  the  rubbed  glass, 
gutta-percha,  or  sealing-wax. 

Nothing  can  be  simpler  than  this  lath 
and  egg  arrangement,  and  hardly  any- 
thing could  be  more  impressive.  The 
more  you  work  with  it,  the  better  you 
will  like  it. 

Pass  an  ebonite  comb  through  the 
hair.  In  dry  weather  it  produces  a 
crackling  noise  ;  but  its  action  upon  the 
lath  may  be  made  plain  in  any  weather. 
It  is  rendered  electrical  by  friction  against 
the  hair,  and  with  it  you  can  pull  the  lath 
quite  round. 

If  you  moisten  the  hair  with  oil,  the 
comb  will  still  be  excited  and  exert  at- 
traction ;  but  if  you  moisten  it  with 
water,  the  excitement  ceases  ;  a  comb 
passed  through  wetted  hair  has  no  power 
over  the  lath.  You  will  understand  the 
meaning  of  this  subsequently. 

After  its  passage  through,  dry  or  oiled 
hair,  balance  the  comb  itself  upon  the 
egg  :  it  is  attracted  by  the  lath.  You 
thus  prove  the  attraction  to  be  mutual : 
the  comb  attracts  the  lath,  and  the  lath 
attracts  the  comb.  Suspend  your  rubbed 
glass,  rubbed  gutta-percha,  and  rubbed 
sealing-wax  in  jour  wire  loep.  They  are 
all  just  as  much  attracted  by  the  lath  as 
the  lath  was  attracted  by  them.  This  is 
an  extension  of  Boyle's  experiment  witk 
the  suspended  amber  (§2). 

IIovv  it  is  that  any  unelectrificd  body 
attracts,  and  is  attracted  by  the  excited 
glass,  sealing-wax,  and  gutta-percha,  we 
shall  learn  by  and  by. 

A  very  striking  illustration  of  electric 
attraction  may  be  obtained  with  the  board 
and  india-rubber  mentioned  in  our  list  of 
materials  (§4).  Place  the  board  before 
the  fire  and  make  it  hot ;  heat  also  a 
sheet  of  foolscr.p  paper  and  place  it  on 
the  board.  There  is  no  attraction  be- 
tween them.  Pass  the  india-rubber  brisk- 
ly over  the  paper.  It  now  clings  firmly 
to  the  board.  Tear  it  away,  arid  hold  jt 
at  arm's  length,  for  it  will  move  to  your 
body  if  it  can.  Bring  it  near  a  door  or 
wall,  it  will  cling  tenaciously  to  either. 
The  electrified  paper  also  powerfully  at- 
tracts tho  balanced  lath  from  a  great  dis- 
tance. 

The  friction  of  the  hnnd,  of  a  cam- 
bric handkerchief,  or  of  wash-leather 
fails  to  electrify  the  paper  in  any  high 
degree.  It  requires  friction  by  a 


233 


LESSONS  IN  ELECTRICITY. 


special  substance  to  make  the  excitement 
strong.  This  \vo  learn  by  experience. 
It  is  also  experience  that  has  taught 
us  that  resinous  bodies  are  best  ex- 
cited by  tianiic-],  and  vitreous  bodies  by 
Bilk; 

Take  nothing-  for  granted  in  this  in- 
quiry, and  neglect  no  effort  to  render 
your  knowledge  complete  and  sure.  Try 
various  rubbers,  r.nd  satisfy  yourself  that 
differences  like  that  first  observed  by 
Newton  exist  between  them. 

Vary  also  the  body  rubbed.  Excite 
by  friction  paraftine  and  composite  can- 
dies, resin,  sulphur,  beeswax,  ebonite, 
and  shellac.  Also  rock-crystal  and  other 
vitreous  substances,  and  attract  with  all 
of  them  the  balanced  lath.  A  film  of 
collodion,  a  sheet  of  vulcanized  india- 
rubber,  or  brown  paper  heated  before 
the  (ire,  rubbed  briskly  with  the  dry 
hand,  attracts  and  is  attracted  by  the 
lath. 

Lay  bare  also  the  true  influence  of 
heat  in  the  case  of  our  rubbed  paper. 
Spread  a  cold  sheet  of  foolscap  on  a  cold 
board — on  a  table,  for  example.  If  the 
air  be  not  very  dry,  rubbing,  even  with 
the  india-rubber,  will  not  make  them 
cling  together.  Bat  is  it  because  they 
were  hot  that  they  attracted  each  other 
in  the  first  instance  ?  No,  for  you  may 
heat  your  board  by  plunging  it  into  boil- 
ing water,  and  your  paper  by  holding  it 
in  a  cloud  of  steam.  Thus  heated  they 
cannot  be  made  to  cling  together.  The 
heat  really  acts  by  expelling  the  moisture. 
Cold  weather,  if  it  be  only  dry,  is  highly 
favorable  to  electric  excitation.  During 
frost  the  whisking  of  the  hand  over  silk 
or  flannel,  or  over  a  cat's  back,  renders 
it  electrical. 

The  experiment  of  the  Florentine 
academicians,  whereby  they  proved  the 
electric  attraction  of  a  liquid,  is  pretty, 
and  worthy  of  repetition.  Fill  a  very 
small  watch-glass  with  oil,  until  the 
liquid  forms  a  round  curved  surface,  ris- 
ing a  little  over  the  rim  of  the  glass.  A 
strongly  excited  glass  tube,  held  over  the 
oil,  raises  not  one  eminence  only,  but 
several,  each  of  which  finally  discharges 
a  shower  of  drops  against  the  attracting 
glass.  The  effect  is  shown  in  fig.  5, 
where  G  is  the  watch-glass  on  the  stand 
T,  and  R  the  excited  glass  tube.* 

Cause  the  excited  glass  tube  to  pass 


FIG.  5. 

close  by  your  face,  without  touching  it. 
You  feel,  like  Hauksbee,  as  if  a  cobweb 
were  drawn  over  the  face.  You  also 
sometimes  smell  a  peculiar  odor,  due  to 
a  substance  developed  by  the  electricity, 
and  called  ozone. 

Long  ere  this,  while  rubbing  your 
tubes,  you  will  have  heard  the  "  hiss- 
ing'' and  *'  crackling"  so  often  referred 
to  by  the  earlier  electricians  ;  and  if  you 
have  rubbed  your  glass  tube  briskly  in 
the  dark,  you  will  have  seen  what  they 
called  the  "electric  fire."  Using,  in- 
stead of  a  tube,  a  tall  glass  jar,  rendered 
hot,  a  good  warm  rubber,  and  vigorous 
friction,  the  streams  of  electric  fire  are 
very  surprising  in  the  dark. 

§  6.   Discovery  of  Conduction  and  Insu- 
lation. 

Here  I  must  again  refer  to  that  most 
meritorious  philosopher,  Stephen  Gray. 
In  1729  he  experimented  with  a  gfoss 
tube  stopped  by  a  cork.  When  the  tubo 
was  rubbed,  the  cork  attracted  light  bod- 
ies. Gray  states  that  he  was  "  much 
surprised  "at  this,  and  he  "  concluded 
that  there  was  certainly  an  attractive  vir- 
tue communicated  to  the  cork."  This 
was  the  starting  point  of  our  knowledge 
of  electric  Conduction. 

A  fir  stick  4  inches  long,  stuck  into 
the  cork,  was  also  found  by  Gray  to  at- 
tract light  bodies.  He  made  his  sticks 

*  As  a  practical  measure  the  watch-glass 
ought  to  rest  upon  a  small  stand,  and  not 
upon  a  surface  of  large  area.  The  experi- 
ment is  particularly  well  suited  for  projection 
on  a  screen. 


S   IX   ELECTRICITY. 


29,1 


longer,  but  f-*,ill  found  a  power  of  attrac- 
tion at  thmi'  ends.  lie  then  passed  on 
to  pack-thread  and  wire.  Hanging  ;i 
thread  s,  fig.  G,  from  the  top  window  of 
a  house,  so  that  the  lower  end  nearly 
touched  the  ground,  and  twisting  the  up- 
per end  of  the  thread  round  his  glass 
tube  R,  on  briskly  rubbing  the  tube,  light 
bodies  were  attracted  by  the  lower  end 
B  of  the  thread. 

But  Gray's  most  remarkable  experi- 
ment was  this  :  IIo  suspended  a  long 
hempen  line  horizontally  by  loops  of  pack- 
thread, but  failed  to  transmit  through 
it  tlm  electric  power.  He  then  suspend- 
ed it  by  ioops  of  silk  and  succeeded  in 


FIG.  6. 

sending  the  "  attractive  virtue"  through 
765  feet  of  thread.  He  at  first  thought 
the  silk  was  effectual  because  it  was  thin  ; 
but  on  replacing  a  broken  silk  loop  by  a 
t-till  thinner  wire,  he  obtained  no  action. 
Finally,  he  came  to  the  conclusion  that 
his  loops  were  effectual,  not  because  they 
wore  thin  but  because  they  were  silk. 
This  was  the  starting-point  of  our  knowl- 
edge of  Insulation. 

It  is  interesting  to  notice  the  devotion 
of  some  men  of  science  to  their  \\<uk. 
Dr.  Wells,  who  wrote  a  beautiful  CHK.-.V, 
wherein  he  explained  the  oriuiu  of  dew, 


finished  it  when  he  was  on  the  brink  of 
the  grave.  Stephen  Gray  was  so  near 
dying  when  his  last  experiments  were 
made,  that  he  was  unable  lo  write  out  an 
account  of  them.  On  his  death-bed, 
and,  indeed,  the  very  day  before  his 
death,  his  description  of  them  was  taken 
from  his  lips  by  Dr.  Mortimer,  Secretary 
of  the  Royal  Society,  and  afterward 
printed  in  the  "  Philosophical  Transac- 
tions." 

One  word  of  definition  will  be  useful 
here.  Some  substances,  as  proved  by 
Stephen  Gray,  possess  in  a  very  high  de- 
gree the  power  of  permitting  electricity 
to  pass  through  them  ;  other  substances 
ptop  the  passage  of  the  electricity. 
Bodies  of  the  first  class  are  called  con- 
ductors ;  bodies  of  the  second  class  arc 
called  insulators. 

You  cannot  do  better  than  repeat  here 
the  experiments  of  Gray.  Push  a  cork 
into  the  open  end  of  your  glass  tube  ; 
rub  the  tube,  carrving  the  friction  up 
to  the  end  holding  the  cork.  The  cork 
will  attract  the  balanced  lath,  shown  in 
fig.  4,  with  which  you  have  already 
\vorked  so  much. 

I5ut  the  excited  glass  is  here  so  near 
the  end  of  the  cork  that  you  may  not 
feel  certain  that  the  observed  attraction  is 
that  of  the  cork.  You  can,  however, 
prove  that  the  cork  attracts  by  its  action 
upon  light  bodies  which  cling  to  it. 
Stick  a  pen-holder  into  the  cork  and  rub 
the  glass  tube  as  before.  The  free  end 
of  the  holder  will  attract  the  lath.  Stick 
a  deal  rod  three  or  four  feet  long  into 
the  cork  ;  its  free  end  will  attract  the 
lath  when  the  glass  tube  is  excited.  In 
this  way  you  prove  to  demonstration 
that  the  electric  power  is  conveyed  along 
the  rod. 

§  7.    The  Electroscope. — Further  Inquir- 
ies on  Conduction  and  Insulation. 

A  little  addition  to  our  apparatus  will 
now  be  desirable.  You  can  buy  a  book 
of  "  Dutch  metal  "  for  fourpence  ;  an  1 
a  globular  flask  like  thst  shown  in  fi.j. 
?,  for  sixpence,  or  at  the  most  a  shilling. 
Find  a  cork,  <:,  which  tin  the  fl  i»k  ;  pn-a 
a  wire,  w,  through  the  cork  an  1  bend  it 
near  one  end  at,  a  right  ati-jfle.  Attach 
by  means  of  w:ix  to  the  bent  arm,  which 
ought  to  be  about  three  (ju.-irters  of  an. 
inch  Jong,  two  strips,  i,  of  the  Dutch 


294 


LESSONS   IN   ELECTRICITY. 


FIG.  7. 

metal,  about  three  inches  long  and  from 
half  an  inch  to  three  quarters  of  an  inch 
wide.  The  strips  will  hang  down  face 
to  face,  in  contact  with  each  other. 
Stic;c  by  sealing-wax  upon  the  other  end  of 
the  wire  a  little  plate  of  tin  or  sheet-zinc, 
T,  about  two  inches  in  diameter.  In  all 
eases  you  must  be  careful  so  to  use  your 
wax  as  not  to  interrupt  the  metallic  con- 
nection of  the  various  parts  of  your  ap- 
paratus, which  we  will  name  an  electro- 
scope. Gold-leaf,  instead  of  Dutch  metal, 
is  usually  employed  for  electroscopes. 
I  recommend  the  "  metal  "  because 
it  is  cheaper,  and  will  stand  rougher 
usage. 

See  that  your  globular  flask  is  dry  and 
free  from  dust.  Bring  your  rubbed  seal- 
ing-wax, R,  or  your  rubbed  glass,  near 
the  little  plate  of  tin,  the  leaves  of  Dutch 
metal  open  out  ;  withdraw  the  excited 
body,  the  leaves  fall  together.  We-  shall 
inquire  into  the  cause  of  this  action  im- 
mediately. Practise  the  approach  and 
withdrawal  for  a  little  time.  Now  draw 
\our  lubbed  sealing-wax  or  glass  along 
the  edge  of  the  tin  plate,  T.  The  leaves 
diverge,  and  after  the  sealing-wax  or 
glass  is  withdrawn  they  remain  divergent. 
In  the  first  experiment  you  communi- 
cated no  electricity  to  the  electroscope  ; 
in  the  second  experiment  you  did.  At 
present  I  will  only  ask  you  to  take  the 
opening  out  of  the  leaves  as  a  proof  that 
electricity  has  been  communicated  to 
them. 


And  now  we  are  ready  for  Gray's  ex- 
periments in  a  form  different  from  his. 
Connect  the  end  of  a  lung  wire  with  the 
tin  plate  of  the  electroscope  ;  coil  the 
other  end  round  your  glass  tube.  Rub 
the  tube  briskly,  carrying  the  friction 
close  to  the  coiled  wire.  A  sino-le  stroke 
of  your  rubber,  if  skilfully  given,  will 
cause  the  leaves  to  diverge.  '  The  elec- 
tricity has  obviously  passed  through  the 
wire  to  the  electroscope. 

Substitute  for  the  wire  a  string  of 
common  twine,  rub  briskly  and  you°will 
cause  the  leaves  to  diVerge  ;  but 'there  is 
a  notable  difference  us  regards  the 
promptness  of  the  divergence.  '.You  soon 
satisfy  yourself  that  the  electricity  ,  a-ses 
with  greater  facility  through  the  wire 
than  through  the  string.  Substitute  for 
the  twine  a  string  of  silk.  No  matter 
how  vigorously  you  rub,  you  can  now 
produce  no  divergence.  The  electricity 
cannot  get  through  the  silk  at  all. 

This  is  the  place  to  demonstrate  in  a 
manner  never  to  be  forgotten  the  influ- 
ence of  moisture.  Wet  your  dry  silk 
string  throughout,  and  squeeze  it  a  little 
so  that  the  water  from  it  may  not  trick1. 3 
over  your  glass  tube.  Coil  it  round  the 
tube  as  before,  and  excite  the  tube.  The 
leaves  of  the  electroscope  immediately 
diverge.  The  water  is  here  the  con- 
ductor. The  influence  of  moisture  was 
first  demonstrated  by  Du  Fay  (1733  to 
1737),  who  succeeded  in  sending  elec- 
tricity through  1256  feet  of  moist  pack- 
thread. 

It  is  hardly  necessary  to  point  out  the 
meaning  of  Gray's  experiment  where  he 
found  that,  with  loops  of  wire  or  of 
pack-thread,  he  could  not  send  the  elec- 
tricity from  end  to  end  of  his  suspended 
string.  Obviously  the  electricity  escaped 
in  each  of  these  cases  through  the  con- 
ducting support  to  the  earth. 

My  assistant,  Mr.  Cotixell,  who  has 
been  working  very  hard  for  you  and  me, 
has  devised  an  electroscope  which  we 
shall  frequently  employ  in  our  lessons. 
M,  fig.  8,  is  a  little  plate  of  metal,  or  of 
wood  covered  with  tin-foil,  supported  on 
a  rod,  G,  of  glass  or  of  sealing-wax.  N 
is  another  plate  of  Dutch  metal  paper, 
separated  about  an  inch  from  M,  and  at- 
tached by  sealing  wax  to  the  long  straw 
i  i'  (broken  off  in  the  figure)  ;  A  A'  is  a 
horizontal  pivot  formed  by  a  sewing 


LESSONS  IN  ELECTRICVfV. 


295 


needle,  and  supported  on  a  bent  strip  of 
mdal,  as  shown  in  the  figure.  By  weight- 
ing  the  straw  with  a  little  wire  near  j', 
you  so  balance  it  that  the  plate  N  shall  be 
just  lifted  away  from  M.  The  wire  w, 
which  may  be  100  feet  long,  proceeds 
from  M  to  your  glass  tube,  round  which 
it  is  coiled.  A  single  vigorous  stroke  of 
the  tube,  by  the  rubber,  sends  electricity 
along  w  to  M  ;  N  is  attracted  downward, 
the  other  end  of  the  long  straw  being 
lifted  through  a  considerable  distance. 
In  subsequent  figures  you  will  see  the 
complete  straw-index,  and  its  modes  of 
application. 

A  few  experiments  with  cither  of  these 
instruments  will  enable  you  to  classify 
bodies  as  conductors,  se'mi-conductors, 
and  insulators.  Here  is  a  list  of  a  few 
of  each,  which,  however,  differ  much 
among  themselves. 

Conductors. 
The  common  metals. 
Well- burned  charcoal. 
Concentrated  acids. 
{Solutions  of  tails. 
Rain  water. 
Li  IK  n. 
Living  vegetables  and  animals. 

Semi-conductor*. 
Alcohol  anJ  ether.         Paper. 
Dry  wood.  8liuw. 

Mmble. 


Insulators. 

Fattv  oils.  Silk. 

Chalk.  Gh;s«. 

India-rubber.  Wax. 

Dry  paper.  Sulphur. 

Haii1.  Shellac. 

A  little  reflection  will  enable  you  to 
vary  these  experiments  indefinitely.  Rul> 
your  excited  sealing-wax  or  glass  ajjf.inst 
the  tin  plate  of  your  electroscope,  and 
cause  the  leaves  to  diverge.  Touch  the 
plate  with  any  one  of  the  conductors 
mentioned  in  the  list  ;  the  electroscope  is 
immediately  discharged.  Touch  it  with 
a  semi-conductor  ;  the  leaves  fail  as  le 
fore,  but  less  promptly.  Touch  the 
plate  finally  with  an  insulator,  the  elt-c- 
tricity  cannot  pass,  and  the  leaves  remain 
unchanged. 

§  8.   Electrics  and  Non- Electrics. 

For  a  long  period,  bodies  were  divided 
into  tlcctrics  and  non-electrics,  the  former 
deemed  capable  of  beinnr  olectriti  >d.  the 
latter  not.  Thus  the  amber  of  the  an- 
cients, and  t\ic  spars,  gems,  fossils,  stones, 
glasses,  and  resins,  operated  on  by  Dr. 
Gilbert,  were  called  electrics,  while  all  the 
metals  were  called  non-electrics.  We 
must  now  determine  the  true  meaning  of 
this  distinction. 

Take  hi  succession  a  piece  of  brass,  of 
wood  coated  with  tin-foil,  a  lead  bullet, 
apples,  pears,  turnips,  carrots,  cucum- 
bers— uncoatcd  wood  not  very  dry  will 
also  answer— in  the  hand,  and  strike  them 
briskly  with  flannel,  or  the  fox's  brush  ; 
none  of  them  will  attract  the  balanced 
lath,  fig.  4,  or  show  any  other  symptom 
of  electric  excitement.  All  of  them 
therefore  would  have  been  onoe  called 
non-electrics. 

But  suspend  them  in  succession  by  a 
string  of  silk  held  in  the  hand,  and  strike 
them  again  ;  every  one  of  them  will  now 
attract  the  lath. 

Reflect  upon  the  meaning  of  this  ex- 
periment. We  have  introduced  an  insu- 
lator— the  silk  string — between  the  hand 
and  the  body  struck,  and  we  find  that  by 
its  introd.iction  the  non-electric  has  been 
converted  into  an  electric. 

The  meaning  is  obvious.  When  held 
in  the  hand,  though  electricity  was  devel- 
oped in  each  case  by  the  friction,  it  pass- 
ed immediately  through  the  hand  and 
body  to  the  caith.  This  transfer  being 
prevented  by  tho  silk,  the  electricity, 


LESSOVS  IN   ELECTRICITY. 


once  excited,  is  retained,  and  the  attrac- 
tion of  the  lath  is  the  consequence. 

In  like  manner,  a  brass  tube,  held  in 
the  hand  and  struck  with  a  fox's  brush, 
shows  no  attractive  power  ;  but  when  a 
stick  of  sealing-wax,  ebonite,  or  gutta- 
percha  is  thrust  into  the  tube  as  a  han- 
dle, the  striking  of  the  tube  at  once  de- 
velops the  power  of  attraction. 

And  now  you  see  more  clearly  than 
you  did  at  first  the  meaning  of  the  ex 
periment  with  the  heated  foolscap  anj 
india-rubber.  Paper  and  wood  always 
imbibe  a  certain  amount  of  moisture 
from  the  air.  When  the  rubber  was 
passed  over  the  cold  paper  electricity  was 
excited,  but  the  paper,  being  rendered  a 
conductor  by  its  moisture,  allowed  the 
electricity  to  pass  away. 

Prove  all  things.  Lay  your  coll  fools- 
cap on  a  cold  board  supported  by  dry- 
tumblers  ;  pass  your  india-rubber  over 
the  paper  ;  lift  it  by  a,  loop  of  silk  which 
has  been  previously  attached  to  it,  for  if 
you  touch  it  it  will  discharge  itself. 
You  will  find  it  electric  ;  and  with  it  you 
can  charge  your  electroscope,  or  attract 
from  a  distance  your  balanced  lath. 

The  human  body  was  ranked  among 
the  non-electrics.  Make  plain  to  your- 
self the  reason.  Stand  upon  the  iloor 
and  permit  a  friend  to  strike  you  briskly 
with  the  fox's  brush.  Present  your 
knuckle  to  the  balanced  lath,  you  will 
find  no  attraction.  Here,  however,  yon. 
stand  upon  the  earth,  so  that  even  if 
electricity  had  been  developed,  there  is 
nothing  to  hinder  it  from  passing  away. 

But,  place  upon  the  ground  four  warm 
glass  tumblers,  and  upon  the  tumblers  a 
board.*  Stand  upon  the  board  and  pre- 
sent your  knuckle  to  the  lath.  A  single 
stroke  of  the  fox's  fur,  if  skilfully  given, 
will  produce  attraction.  If  you  stand 
upon  a  cake  of  resin,  of  ebonite,  or  upon 
a  sheet  of  good  •  india-rubber,  the  effect 
will  be  the  same.  You  can  also  charge 
your  electroscope  with  this  electricity. 

Throw  a  mackintosh  over  your  shoul- 
ders and  let  a  friend  strike  it  with  the 
fox's  brush,  the  attractive  force  is  greatly 
augmented. 

After    brisk     striking,     present    your 

*  Some  caution  is  necessary  hero.  A  large 
class  of  cheap  glass  tumblers  conduct  s.i 
freely  that  they  are  unfit  for  this  and  similar 
expeiiineiits.  See  §  19. 


knuckle  to  the  knuckle  of  your  friend. 
A  spark  will  pass  between  you. 

This  experiment  with  the  mackintosh 
further  illustrates  what  you  have  already 
frequently  observed — namely,  that  it  i> 
not  friction  alono,  but  the  friction  of 
special  substances  against  each  other,  that 
produces  electricity. 

Thus  we  prove  that  non-electrics,  like 
electrics,  can  be  excited,  the  condition  of 
success  being,  that  an  insulator  shall  be 
interposed  between  the  non-electric  and 
the  earth.  It  is  obvious  that  the  old  di- 
vision into  electrics  and  non-electrics, 
really  meant  a  division  into  insulators 
and  conductors. 

§  9.   Electric  Repulsions. — Discovery  of 
two  Electricities. 

Wo  have  hitherto  dealt  almost  exclu- 
sively with  electric  attractions,  but  in  an 
experiment  already  referred  to  (§2), 
Otto  von  Guericke  observed  the  repulsion 
of  a  feather  by  his  sulphur  globe.  I  also 
anticipated  mutters  in  the  use  of  our 
Dutch  metal  electroscope  (§  7),  where 
the  repulsion  of  the  leaves  informed  us 
of  the  arrival  of  the  electricity. 

Du  Fay,  who  was  the  real  discoverer 
here,  found  a  gold-leaf  floating  in  the  air 
to  be  first  attracted  and  then  repelled  by 
the  same  excited  body.  He  afterward 
proved  that  when  -the  floating  leaf  was 
repelled  by  rubbed  glass,  it  was  attracted 
by  rubbed  resin — and  that  when  it  was 
repelled  by  rubbed  resin  it  was  attracted 
by  rubbed  glass.  Hence  the  important 
announcement,  by  Du  Fay,  that  there  are 
two  kinds  of  electricity. 

The  electricity  excited  on  glass  was  for 
a  time  called  vitreous  electricity,  while 
that  excited  on  sealing-wax  was  called  res- 
inous electricity.  These  terms  are  how- 
ever improper  ;  because,  by  changing  the 
rubber,  we  can  obtain  the  electricity  of 
sealing-wax  upon  glass,  and  the  electric- 
ity of  glass  upon  sealing-wax. 

Roughen,  for  example,  the  surface  of 
your  glass  tube  with  a  grindstone,  and 
rub  it  with  flannel,  the  electricity  of  seal- 
ing-wax will  be  found  upon  the  vitreous 
surface.  Rub  your  sealing-wax  with  vul- 
canized india-rubber,  the  electricity  of 
glass  will  be  found  upon  the  resinous  sur- 
face. You  will  be  able  to  prove  this  im- 
mediately. 


LESSONS  IN  ELECTRICITY. 


897 


We  now  use  the  term  positive  or  plus 
electricity  to  denote  that  developed  on 
glass  by  the  friction  of  silk  ;  and  negative 
or  minus  electricity  to  denote  that  devel- 
oped on  sealing-wax  by  the  friction  of 
flannel.  These  terms  are  adopted  purely 
for  the  sake  of  convenience.  There  is 
no  reason  in  nature  why  the  resinous 
electricity  should  not  be  called  positive, 
and  the  vitreous  electricity  negative. 
Once  agreed,  however,  to  apply  the  terms 
as  here  fixed,  we  must  adhere  to  this 
agreement  throughout. 

§   10.   Fundamental  Law  of  Electric 
Action. 

In  all  the  experiments  which  we  have 
hitherto  made,  one  of  the  substances  op- 
erated on  has  been  electrified  b  friction, 
and  the  other  not.  But  once  engaged  in 
inquiries  of  this  description,  questions 
incessantly  occur  to  the  mind,  the  an- 
swering of  which  extends  our  knowledge 
and  suggests  other  questions.  Suppose, 
instead  of  exciting  only  one  of  the  bod- 
ies presented  to  each  other,  we  were  to 
vxcite  both  of  them,  what  would  occur  ? 
This  is  the  question  which  was  asked  and 
answered  by  Du  Fay,  and  which  wo  must 
now  answer  for  ourselves. 

Here  your  wire  loop,  fig.  1,  comes 
again  into  play.  Place  an  unrubbed 
gutta-percha  tube,  or  a  stick  of  sealing- 
wax,  in  the  loop,  and  be  sure  that  it  is 
unrubbed — that  no  electricity  adheres  to 
it  from  former  experiments.  If  it  fail  to 
attract  light  bodies,  it  is  unexcited  ;  if  it 
attract  them,  pass  your  hand  over  it  sev- 
eral times,  or,  better  still,  pass  it  over 
or  through  the  flame  of  a  spirit  lamp. 
This  will  remove  every  trace  of  electric- 
ity. Satisfy  yourself  that  the  unrubbed 
gutta-percha  tube  is  attracted  by  a  rubbed 
one. 

Remove  the  unrubbed  tube  from  the 
loop,  and  excite  it  with  its  flannel  rub- 
ber. One  end  of  the  tube  is  held  in  your 
hand  and  is  therefore  unexcited.  Return 
ihe  tube  to  the  loop,  keeping  your  eye 
upon  the  excited  end.  Bring  a  second 
rubbed  tube  near  the  excited  end  of  the 
suspended  one  :  strong  repulsion  is  the 
consequence.  Drive  the  suspended  tube 
round  and  round  by  this  force  of  repul- 
sion 

Bring  a  rubbed  glass  tube  near  the  ex- 
cited end  of  the  gutta-percha  tube : 
strong  attraction  is  the  result. 


Repeat  this  experiment  step  by  step 
with  two  glass  tubes.  Prove  that  the 
rubbed  glass  tube  attracts  the  unrubbed 
one.  Remove  the  unrubbed  tube  from 
the  loop,  excite  it  by  its  rubber,  return  it 
to  the  loop,  and  establish  the  repulsion 
of  glass  by  glass.  Bring  rubbed  gutta- 
percha  or  sealing-wax  near  the  rubbed 
glass  :  strong  attraction  is  the  conse- 
quence. 

These  experiments  lead  you  directly  to 
the  fundamental  law  of  electric  action, 
which  is  this  :  Bodies  charged  with  the 
same  electricity  repel  each  other,  while 
bodies  charged  with  opposite  electricities 
attract  each  other.  Positive  repels  posi' 
five,  and  attracts  negative.  Negative 
repels  negative  and  attracts  positive.^ 

Devise  experiments  which  shall  still 
further  illustrate  this  law.  Repeat,  for 
example,  Otto  von  Guericke's  experi- 
ment. Hang  a  feather  by  a  silk  thread 
and  bring  your  rubbed  glass  tube  near  it  : 
the  feather  is  attracted,  touches  the 
tube,  charges  itself  with  the  electricity  of 
the  tube,  and  is  then  repelled.  Cause  it 
to  retreat  from  the  tube  in  various  direc- 
tions. Du  Fay's  experiment  with  the 
gold-leaf  will  be  repeated  and  explained 
further  on.  See  §  18. 

Hang  your  feather  by  a  common 
thread  ;  if  no  insulating  substance  inter- 
venes between  the  feather  and  the  earth, 
you  can  get  no  repulsion.  Why  ?  Ob- 
viously because  the  charge  of  positive 
electricity  communicated  by  the  rod  is 
not  retained  by  the  feather,  but  passes 
away  to  the  earth.  Hence,  you  have  not 
positive  acting  against  positive  at  all. 
Why  the  neutral  body  is  attracted  by 
the  electrified  one,  will,  as  already 
stated,  appear  by  and  by. 


PIG.  11. 


LESSOKS 


Attract  your  straw  needle  by  your  rub- 
bed  glass  tube.  Let  the  straw  strike  the 
tul>e,  so  that  the  one  shall  rub  against 
the  other.  The  straw  accepts  the  elec- 
tricity of  the  tube,  and  repulsion  immedi- 
ately follows  attraction,  as  shown  in  fig. 
9. 

Mr.  Cuttrcll  has  devised  the  simple 
electroscope  represented  in  fig.  10  to 
snow  repulsion.  A  is  a  stem  of  sealing- 
wax  with  a  small  circle  of  tin,  T,  at  the 
top.  w  is  a  bent  wire  proceeding  from 
T,  with  a  small  disk  attached  to  it  by 
wax.  i  i'  is  a  little  straw  index,  support- 
ed by  the  needle  N,  as  shown  in  fig.  10. 
The  stem  A',  also  of  sealing-wax,  is  not 
quite  vertical,  the  object  being  to  cause 
the  bit  of  paper,  i',  to  rest  close  to  w 
when  the  apparatus  is  not  electrified. 
When  electricity  is  imparted  to  T,  it 
f.ows  through  the  wires  w  and  w,  over 
both  disk  and  index  :  immediate  repul- 
sion of  the  straw  is  the  consequence. 

No  better  experiment  can  be  made  to 
illustrate  the  self-repulsive  character  of 
electricity  than  the  following  one.  Heat 
your  square  board  (§  5),  and  warm,  as 
before,  your  sheet  of  foolscap.  Spread 
the  paper  upon  the  board,  and  excite  it 
by  the  friction,  of  india-rubber.  Cut 
from  the  sheet  two  long  strips  with  your 


penknife.  Hold  the  stiips  together  at 
one  end.  Separate  them  from  the  board, 
and  lift  them  into  the  air  :  they  forcibly 
diive  each  other  apart,  producing  a  wide 
divergence. 

Cut  r.-.rveral  stiips.  so  as  to  form  a  kind 
of  tassel.  Hold  them  together  at  one 
end.  Separate  them  from  the  board,  and 
lift  them  into  the  air  :  they  are  driven 
asunder  by  the  self-repellent  electricity, 
presentini*"  an  appearance  which  may  ie- 
mind  you  of  the  hair  of  Medusa.  The 
effect  is  represented  in  fig.  11.* 

Another  very  beautiful  experiment  fits 
in  here.  Let  fine  silver  sand,  s,  fig.  12, 
issue  in  a  stream  from  a  glass  funnel, 
through  an  aperture  one  eighth  of  an  inch 
in  diameter.  Connect  the  sand  in  the 
funnel  by  a  fine  wire  w,  fig.  13,  with  your 
warm  g-lass  tube.  Unelectrified,  the 


*  In  one  of  my  earliest  lectures  at  the 
Royal  Institution,  having  rubbed  a  sheet  of 
foulscap,  I  was  about  lo  lii't  it  bodily  from 
the  hot  board,  and  to  place  it  against  the 
wall,  when  the  thought  of  cutting  it  into 
strips,  and  allowing  them  to  net  upon  each 
other,  orciirnd  to  me.  The  itsult,  of 
course,  was  that  above  described.  Simple 
and  obvious  as  it  was.  it  gave  Faraday,  who 
was  present  at  the  time,  the  most  lively 
pleasure.  The  simplest  experiment,  if  only 
suited  to  its  object,  delighted  him. 


LESSONS   IN   ELECTRICITY. 


299 


FIQ.  12. 


FIG.  11. 


RflTKl  particles  descend  as  a  continuous 
si i cam,  s  s',  fig.  12,  but  at  every  stroke 
of  die  rubber  they  fly  asunder,  as  in  fig. 
13,  through  self-repulsion. f 

Or  let  three  or  four  fine  fillets  of  water 
issue  from  three  or  four  pin-holos  in  the 
bottom  of  a  vessel  close  to  each  other. 
Connect  the  water  of  the  vessel  with 
your  glass  tube,  and  rub  as  before.  The 
liquid  veins  are  scattered  into  spray  by 
every  stroke  of  the  rubber. 

These  experiments  are  best  made  with 
"  CottrelPs  rubber,"  described  in  §  24. 

And  now  you  must  learu  to  determine 
with  certainty  the  quality  cf  the  elec- 
tricity with  which  any  body  presented  to 
you  may  be  charged.  You  see  immedi- 
ately that  attraction  is  no  sure  test,  be- 
cause unelectrified  bodies  are  attracted. 
Further  on  (§  14)  you  will  be  able  to 
grapple  with  another  possible  source  of 
error  in  the  employment  of  attraction. 

In  determining  quality,  you  must 
ascertain,  by  trial,  the  kind  of  electricity 
by  which  the  charged  body  is  repelled  ; 
if,  for  example,  any  electrified  body  re- 
pel, or  is  repelled  by,  sealing-wax  rubbed 
with  flannel,  the  electricity  of  the  body 

f  For  these,  and  also  for  experiments  with 
thu  electroscope,  the  teacher  of  a  large  class 
will  find  the  lime  light  shadows  upon  awhile 
screen  (or  better  still,  those  of  the  electric 
light)  exceedingly  useful.  The  effects  arc 
thus  rendered  visible  to  all  at  once. 


is  negative  ;  if  it  repel,  or  is  repelled  by, 
glass,  rubbed  with  silk,  its  electricity  is 
positive.  Du  Fay  had  the  sagacity  to 
propose  this  mode  of  testing  quality. 

Apply  this  test  to  the  strips  of  fools- 
cap paper  excited  by  the  india-rubber. 
Bring  a  rubbed  gutta-percha  tube  near 
the  electrified  strips,  you  have  strong  at- 
traction. Bring  a  rubbed  glass  tube 
between  the  strips,  you  have  strong  re- 
pulsion and  augmented  divergence. 
Hence,  the  electricity,  being  repelled  by 
the  positive  glass,  is  itself  positive. 

§11.  Electricity  of  the  Rubber. — Doubh 
or  "  Polar"  Character  of  the  Electric 
Force. 

We  have  examined  the  action  of  each 
kind  of  electricity  upon  itself,  and  upon 
the  other  kind  ;  but  hitherto  we  have 
kept  the  rubber  out  of  view.  One  of 
the  questions  which  inevitably  occur  to 
the  inquiring  scientific  mind  would  be, 
How  is  the  rubber  affected  by  the  act  of 
friction  ?  Here,  as  elsewhere,  you  must 
examine  the  subject  for  yourself,  and 
base  your  conclusions  on  the  facts  you 
establish. 

Test  your  rubber,  then,  by  your  bal- 
anced lath.  The  lath  is  attracted  by  the 
flannel  which  has  rubbed  against  gutta- 
percha  ;  and  it  is  attracted  by  the  silk, 
which  has  rubbed  against  glass. 


300 


LESSOMS   IN   FLTCTRTCITY. 


Regarding  the  quality  of  the  electricity 
of  the  flannel  or  of  the  silk  rubber,  the 
attraction  of  the  lath  teaches  you  nothing. 
But,  suspend  your  rubbed  glass  tube,  and 
bring  the  flannel  rubber  near  it  :  repul- 
sion follows.  The  silk  rubber,  on  the 
contrary,  attracts  the  glass  tube.  Sus- 
pend your  rubbed  gutta-percha  tube,  and 
bring  the  silk  rubber  near  it  ;  repulsion 
follows.  The  flannel,  on  the  contrary, 
attracts  the  tube. 

The  conclusion  is  obvious  :  the  elec- 
tricity of  the  flannel  is  positive,  that  of 
the  silk  is  negative. 

But  the  flannel  is  the  rubber  of  the 
gutta-percha,  whose  electricity  is  nega- 
tive ;  and  the  silk  is  the  rubber  of  the 
glass,  whose  electricity  is  positive.  Con- 
sequently, wo  have  not  only  proved  the 
rubber  to  be  electrified  by  the  friction, 
but  also  proved  the  electricity  of  the 
rubber  to  be  opposite  in  quality  to  that 
of  the  body  rubbed. 

All  your  subsequent  experience  will 
verify  the  statement  that  the  two  electric- 
ities always  go  together  ;  that  you  can- 
not excite  one  of  them  without  at  the 
wine  time  exciting  the  other,  and  that 
the  lectricity  of  the  rubber,  though  op- 
posite in  quality,  is  in  all  cases  precisely 
equal  in  quantity  to  that  of  the  body 
rubbed. 

And  now  we  will  test  these  principles 
by  a  new  experiment.  In  §  5  we  learned 
that  an  ebonite  comb  is  electrified  by  its 
passage  through  dry  hair.  You  can 
readily  prove  the  electricity  of  the  comb 
to  be  negative.  But  the  hair  is  here  the 
rubber,  and,  in  accordance  with  the 
principle  just  laid  down,  an  equal  quan- 
tity of  positive  electricity  has  been  excit- 
ed in  the  hair.  If  you  stand  on  the 
Ground  uninsulated,  the  electricity  of  the 
hair  passes  freely  through  your  body  to 
the  earth. 

But  stand  upon  an  insulating  stool* — 
on  your  board,  for  example,  supported 
by  four  warm  tumblers — while  I,  stand- 
ing on  the  ground,  pass  the  comb  briskly 
through  your  hair.  I  pass  it  ten,  twenty, 
tliiity  times,  and  then  ask  you  to  attract 

*  A  stool  with  glass  legs  wnieh,  to  protect 
them  f:om  the  moisture  of  the  air.  are 
usually  coated  with  a  solution  of  shellac. 
Regarding  the  attraction  of  glass  for  atmos- 
pheric humidity,  you  will  call  to  mind  what 
Las  been  said  in  §  5. 


your  balanced  lath.  You  present  your 
knuckle,  but  there  is  no  attraction. 

Here  the  comb  and  the  hair  soon  reach 
their  maximum  excitement,  bevond  which 
no  further  development  of  electricity  oc- 
curs. Now,  though  the  comb,  as  shown 
in  §  5,  is  competent  to  attract  the  lath, 
while  your  body  is  here  incompetent  to 
do  so,  this  may  be  because  the  small 
quantity  of  electricity  existing  in  a  con- 
centrated form  upon  the  comb  becomes, 
when  dirfused  over  the  body,  too  feeble 
to  produce  attraction. 

Can  we  not  exalt  the  electricity  of  your 
body  ?  Guided  by  the  principles  laid 
down,  let  us  try  to  do  so.  First  I  pass 
the  unelectrified  comb  through  your 
hair  ;  it  comes  away  electrified.  After 
discharging  the  comb  by  passing  my  hand 
closely  over  it,  I  pass  it  again  through 
the  hair.  As  before,  it  quits  the  hair 
electrified,  and  I  again  discharge  it.  I 
do  this  ten  or  twenty  times,  always  de- 
priving the  comb  of  its  electricity  after  it 
has  quitted  the  hair.  Now  present  your 
knuckle  to  the  balanced  lath.  It  is  pow- 
erfully attracted. 

Here,  as  I  have  said,  the  unelectrified 
comb  carried  in  each  case  electricity  away 
with  it  ;  but,  in  accordance  with  the  fore- 
going principles,  it  left  an  equal  quantity 
of  the  opposite  electricity  behind  it. 
And  though  the  amount  of  electiicity 
corresponding  to  a  single  charge  of  the 
comb,  when  diffused  over  the  body, 
proved  insensible  to  our  tests,  that 
amount  ten  or  twenty  times  multiplied 
became  not  only  sensible,  but  strong. 
Indeed,  by  discharging  the  comb,  and 
passing  it  in  each  case  undeetrified 
through  the  hair,  the  insulated  human 
body  cair  be  rendered  highly  electrical. 

Near  the  beginning  of  this  section  I 
said,  in  rather  an  off-hand  way,  thai 
rubbed  flannel  repels  rubbed  glass,  while 
rubbed  silk  repels  rubbed  gutta-percha. 
Now,  while  it  is  generally  easy  to  obtain 
the  repulsion  by  the  flannel,  It  is  by  no 
means  always  easy  to  obtain  the  repulsion 
by  the  silk.  Over  and  over  again  I  have 
been  foiled  in  my  attempts  to  show  this 
repulsion.  I  wish  you,  therefore,  to  be 
awaro  cf  an  infallible  method  of  obtain- 
ing it. 

Stand  on  your  insulating  stool,  and  rub 
your  glass  tube  briskly  with  the  amalga- 
mated silk  ;  hand  me  the  tafcc.  I  pass 


LESSONS  IN  ELECTRICITY. 


301 


my  hand  closely  over  its  surface,  rcmor- 
iug  from  that  surface  nearly  the  whole  of 
its  electricity.  I  hand  you  the  tube 
again,  and  you  again  excite  it.  You 
hand  it  to  me,  and  I  again  discharge  it. 
In  each  case,  therefore,  you  excite  an  un- 
clectrified  glass  tube,  and  in  each  case 
the  tube  leaves  behind  upon  the  rubber 
an  amount  of  negative  electricity  equal  in 
quantity  to  the  positive  carried  away. 
By  thus  adding  charge  to  charge,  the 
rubber  is  rendered  highly  electrical  ;  and 
even  should  its  insulating  power  bo  im- 
paired by  the  amalgam,  it  can  now  afiord 
to  yield  a  portion  of  its  electricity  to  your 
hand  and  body,  and  still  powerfully  repel 
rubbed  gutta-percha.  The  principle, 
which  might  be  further  illustrated,  is  ob- 
tiously  the  same  as  that  applied  in  the 
case  of  the  comb. 

§12.    What  is  Electricity  ? 

Thurs  far  we  have  proceeded  from  fact 
(o  fact,  acquiring  knowledge  of  a  very 
valuable  kind.  But  facts  alone  cannot 
satisfy  us.  We  seek  a  knowledge  of  the 
principles  which  lie  behind  the  facts,  and 
which  aie  to  be  discerned  by  the  mind 
alone.  Thus,  having  spoken  as  we  have 
done,  of  electricity  passing  hither  and 
thither,  and  of  its  being  prevented  from 
passing,  hardly  any  thoughtful  boy  cr 
giri  can  avoid  asking  what  is  it  that  thus 
passes  ? — what  is  electricity  ?  Boyle  and 
Newton  betrayed  their  need  of  an  answer 
to  this  question  when  the  one  imagined 
his  unctuous  threads  issuing  from  and  re- 
turning to  the  electrified  body  ;  and  \\hen 
the  other  imagined  that  an  elastic  fluid 
existed  which  penetrated  his  rubbed  glass. 

When  I  say  "  imagined  "  I  do  not  in- 
tend to  represent  the  notions  cf  these 
great  mew  as  vain  fancies.  "Without  im- 
agination we  can  do  nothing  here.  By 
imagination  I  mean  the  power  of  pictur- 
ing mentally  things  which,  though  they 
have  an  existence  as  real  as  that  of  the 
world  around  us,  cannot  be  touched 
directly  by  the  organs  of  sense.  I  mean 
the  purified  scientific  imagination,  with- 
out the  exercise  of  which  we  cannot  take 
a,  single  step  into  the  region  of  causes  and 
principles. 

It  was  by  the  exercise  of  the  scientific 
imagination  that  Franklin  devised  the 
theory  of  a  single  electric  fluid  to  explain 
electrical  phenomena.  This  fluid  he  sup- 


posed to  be  self-repulsive,  and  diffused 
in  definite  quantities  through  all  bodies. 
lie  supposed  that  when  a  body  has  more 
than  its  proper  share  it  is  positively, 
when  less  than  its  proper  share  it  is  nega- 
tively electrified.  It  was  by  the  exercise 
of  the  same  faculty  that  Symmer  devised 
the  theory  of  two  electric  fluids,  each 
self-repulsive,  but  both  mutually  attrac- 
tive. 

At  first  sight  Franklin's  theory  seems 
by  far  the  simpler  of  the  two.  But  its 
simplicity  is  only  apparent.  For  though 
Franklin  assumed  only  one  fluid,  he  was 
obliged  to  assume  three  distinct  actions. 
Firstly,  he  had  the  self -repulsion  of  the 
electric  particles.  Secondly,  the  mutual 
atraction  of  the  electric  particles  and  the 
oonderablc  particles  of  the  body  through 
which  the  ciectnoitv  was  diffused. 
Thirdly,  these  two  assumptions  when 
strictly  followed  out  ica^  10  the  unavoid- 
able conclusion  that  the  material  particles 
also  mutually  repel  each  other.  Thus  the 
theory  is  by  no  means  so  simple  as  it  ap- 
pears. 

The  theory  of  Symmer,  though  at  first 
pight  the  most  complicated,  is  in  reality 
by  far  the  simpler  of  the  two.  Accord- 
ing to  it  electrical  actions  are  produced 
by  two  fluids,  each  .self-repulsive,  but 
both  mutually  attractive.  These  fluids 
cling  to  the  atoms  of  matter,  and  carry 
the  matter  to  which  they  cling  along  with 
them.  Every  body,  in  its  natural  condi- 
tion, possesses  both  fluids  in  equal  quan- 
tities. As  long  {is  the  fluids  are  mixed 
together  they  neutralize  each  other,  the 
body  in  which  they  arc  thus  mixed  being 
in  its  natural  or  unelectrical  condition. 

By  friction  (and  by  various  other 
means)  these  two  fluids  may  be  torn 
asunder,  the  one  clinging  by  preference  to 
the  rubber,  the  other  to  the  body  rubbed. 

According  to  this  theory  there  must 
always  be  attraction  between  the  rubber 
and  the  body  rubbed,  because,  as  we 
have  proved,  they  arc  oppositely  electri- 
fied. This  is  in  fact  the  case.  And 
mark  what  I  now  say.  Over  and  above 
the  common  friction,  this  electrical  at- 
traction has  to  be  overcoaao  v/fene^r<  wa 
rub  glass  with  silk,  cr  88fiiL^g~»tfi*  .^itli. 
flannel. 

You  arc  too  young  to  fully  grasp  this 
subject  yet  ;  and  indeed  it  would  lead  us 
too  far  away  to  enter  fully  into  it.  But 


802 


LESSORS  IN  ELECTRICITY. 


I  will  throw  out  for  future  reflection  the 
remark,  that  the  overcoming  of  the  ordi- 
nary friction  produces  heat  then  and  there 
upon  the  surfaces  rubbed,  while  the  force 
expended  in  overcoming  the  electric  at- 
traction may  be  converted  into  heat 
which  sLal!  appear  a  thousand  miles  away 
from  the  place  whore  it  was  generated. 

Theoretic  conceptions  are  incessantly 
checked  and  corrected  by  the  advance  of 
knowledge,  and  this  theory  of  clectiic 
fluids  is  doubted  by  many  eminent  scien- 
tific men.  It  will,  at  till  events,  have  to 
be  translated  into  a  form  which  shall  con- 
nect it  with  heat  and  light,  before  it  can 
be  accepted  as  complete.  Nevertheless, 
keeping  ourselves  unpledged  to  the  the- 
ory, we  shall  find  it  of  exceeding  service 
both  in  unravelling  and  in  connecting  to- 
gether electrical  phenomena. 


§  13.   Electric  Induction, 
the  Term. 


Definition,  of 


We  have  now  to  apply  the  theory  of 
electric  fluids  to  the  important  subject  of 
electric  induction. 

It  was  noticed  by  early  observers  that 
contact  was  not  necessary  to  electrical  ex- 
citement. Otto  von  Gueiickc,  as  we 
have  already  seen  (§  2),  found  that  a 
body  brought  near  his  sulphur  globe  be- 
came electrical.  By  bringing  his  excited 
glass  tube  near  one  end  of  a  conductor, 
Stephen  Gray  attracted  light  bodies  at 
the  other  end.  lie  also  obtained  attrac- 
tion through  the  human  body.  From 


FIG.  15. 

the  human  body  also  Du  Fay,  to  his 
astonishment,  obtained  a  spark.  Can- 
ton, in  1753,  suspended  pith-balls  by 
thread,  and  holding  an  excited  glass 
tube,  at  a  considerable  distance  from 
them,  caused  them  to  diverge.  On  re- 
moving the  tube  the  balls  fell  together, 
no  permanent  charge  being  impaited  to 
them.  Such  phenomena  were  further 
studied  i:nd  developed  by  Wilcke  and 
^pinus,  Coulomb  and  Poisson. 

These  and  all  similar  results  are  cm- 
braced  by  the  law,  that  when  an  electri- 
fied body  is  brought  near  an  unelectrified 
conductor,  the  neutral  fluid  of  the  latter 
is  decomposed  ;  one  of  its  constituents 
being  attracted,  the  other  repelled. 
When  the  electrified  body  is  withdrawn, 
the  separated  electricities  flow  again  to- 
gether and  render  the  conductor  unelec- 
tric. 

This  decomposition  of  the  neutral  fluid 
by  the  mere  presence  of  an  electrified 
body  is  called  induction.  It  is  also  called 
electrification  by  influence. 

If,  whilf  't  is  under  the  influence  of 
the  electrified  bodv,  the  bodv  influenced 
be  touched,  the  free  electricity  (which  is 
always  of  the  same  kind  as  that  of  the 
influencing  body)  passes  away,  the 
opposite  electricity  being  held  cap- 
tive. 

On  removing  the  electrified  body  the 
captive  electricity  is  set  free,  the  conduc- 
tor being  charged  with  electricity  oppo- 
site in  kind  to  that  of  the  body  which 


^LESSOXS  IN  ELECTRICITY. 


303 


electrified  it. 

You  cannot  do  better  lierc  than  repeat 
Stephen  Gray's  experiment.  Suppoit  :i 
small  plank  or  lath,  L  L',  Fig.  14,  upon  a 
wnnn  tumbler,  G,  and  bring  under  one  of 
its  ends,  L,  and  within  four  or  five  inches 
of  that  end,  scraps  of  light  paper  or  of 
gold  leaf.  Excite  your  glass  tube,  R, 
vigorously,  and  bring  it  over  the  other 
end  of  the  plank,  without  touching  it. 
The  ends  may  be  six  or  eight  feet  apart  ; 
the  light  bodies  will  be  attracted.  The 
experiment  is  easily  made,  and  you  are 
not  to  rust  satisfied  till  you  can  make  it 
with  case  and  certainty. 

This  is  a  fit  place  to  repeat  that  you 
must  keep  a  close  eye  upon  the  tumblers 
you  employ  for  insulation.  Some  of 
thorn,  made  of  common  glass,  are  hardly 
to  be  accounted  insulators  at  all. 

§  14.   Experimental  Researches  on  2Jlcc- 
tric  Induction. 

Our  mastery  over  this  subject  of  in- 
duction must  be  complete  ;  for  it  under- 
lies all  o';r  subsequent  inquiries.  "With- 
out reference  to  it  nothing  is  to  be  ex- 
plained ;  possessed  of  it  you  will  enjoy 
not  only  a  wonderful  power  of  explana- 
tion, but  of  prediction.  We  will  attack 
it,  therefore,  with  the  determination  to 
exhaust  it. 

Arid  here  a  slight  addition  must  be 
made  to  our  apparatus.  We  must  be  in 
a  condition  to  take  samples  of  electricity, 
and  to  convey  them,  with  the  view  of 
testing  them,  from  place  to  place.  For 
this  purpose  the  little  "carrier,"  shown 
in  fig.  15,  will  be  found  convenient.  T 
i.5  a  bit  of  tin-toil,  two  or  three  inches 
square.  A  9*. raw  stem  is  stuck  on  to  it 
by  sealing-wax,  the  lower  end  of  the  stem 
being  covered  by  sealing-wax.  To  make 
the  inflation  sure,  the  part  between  R 
and  s'  is  wholly  of  sealing-wax,  You 
v,an  have  sterns  of  ebonite,  which  are 
stronger,  for  a  few  pence  ;  but  you  can 
have  this  one  for  a  fraction  of  a  penny. 
The  end  u'  is  to  be  held  in  the  hand  ;  the 
electrified  body  is  to  be  touched  by  T, 

id  the  electricity  conveyed  to  an  clec- 

osoopo  to  be  tested. 
Touch  your  rubbed  glass  rod  with  T, 

id  then  touch  your  electroscope  :  the 
leaves  diverge  with  positive  electricity. 
~?oacli  your  rubbed  gutta-percha  or  seal- 
ing-wax \vitIiT,  and  then  touch  your  elec- 


troscope :  the  leaves  diverge  with  nega- 
tive electricity.  If  the  electricity  of  any 
body  augment  the  divergence  produced 
by  the  glass,  the  electricity  of  that  body 
is  positive.  If  it  augment  the  divergence 
produced  by  the  gutta-percha,  the  elec- 
tricity is  negative.  And  now  we  ar^ 
ready  for  further  work. 

Place  an  egg,   E,   fig.  16,   on  its  sid* 


FIG.  16. 

upon  a  dry  wine-glass  \  bring  your  ciciV- 
cd  glass  tube,  o,  within  an  inch  or  so  of 
the  end  of  the  egg.  What  i.s  the  condi- 
tion of  the  egg  1  Its  electricity  is  decom- 
posed ;  the  negative  fluid  covering  the 
end  a  adjacent  to  the  glass,  the  positive 
covering  the  other  end  b.  Remove  the 
glass  tube  :  what  occurs  ?  The  two  elec- 
tricities flow  together  and  neutrality  is 
restored.  Prove  this  neutrality.  Neither 
a  carrier  touching  the  egg,  nor  the  egg  it- 
self, has  any  power  to  pffect  your  elec- 
troscope, or  to  attract  your  balanced  lath. 
Again,  bring  the  excited  tube  near  tho 
egg.  Touch  its  distant  part  b  with  your 
carrier.  The  carrier  now  attracts  the 
straw  (fig.  2)  or  the  balanced  lath  (fig.  4). 
It  also  causes  tho  leaves  of  your  electro- 
scope to  diverge.  What  is  the  quality 
of  the  electricity  ?  It  repel*  and  i-i  rc- 

rlled  by  rubbed  glass  ;  the  electricity  at 
is  therefore  positive.  Discharge  tho 
carrier  by  touching  it,  and  bring  it  into 
contact  with  the  end  a  of  the  cg.-j  nearest 
to  the  glass  tube.  The  electricity  you 
take  away  repels  and  \s>  repelled  by  gutta- 
percha.  It  is  therefore  negative.  Test 
the  quality  also  by  the  electroscope. 

While  the  tube  o  is  near  the  egg  touch 
the  end  b  with  your  finger  ;  now  try  to 
charge  the  carrier  by  touching  b  :  you 
cannot  do  so — the  positive  electricity  has 
disappeared*  Has  the  negative  disap- 


804 


LESSONS  IN  ELECTRICITY. 


peared  also  ?  No.  Remove  the  glass 
tube,  and  once  more  touch  the  egg  at  b 
by  the  carrier.  It  is  charged,  not  with 
positive,  but  with  negative  electricity. 
nearly  understand  this  experiment.  The 
neutral  electricity  of  the  egg  is  first  de- 
composed into  negative  and  positive  ;  the 
former  attracted,  the  latter  repelled  by 
the  excited  glass.  The  repelled  elec- 
tricity is  free  to  escape,  and  it  has  escaped 
on  your  touching  the  egg  with  your  fin- 
ger. But  the  attracted  electricity  cannot 
escape  as  long  as  the  influencing  tube  is 
held  near.  On  removing  the  tube  which 
holds  the  negative  fluid  in  bondage,  that 
fluid  immediately  diffuses  itself  over  the 
whole  egg.  An  apple,  or  a  turnip,  will 
answer  for  these  experiments  at  least  as 
well  as  an  egg. 

Discharge  the  egg  by  touching  it.  Rc- 
cxcite  the  glass  tube  and  bring  it  again 
near.  Touch  the  egg  wth  a  wire  or  with 
your  finger  at  a.  Is  it  the  negative  at  a, 
into  which  you  plunge  your  finger, 
tlutt  escapes  ?  No  such  thing.  The  free 
positive  fluid  passes  through  the  nega- 
tive, and  through  your  finger  to  the 
earth.  Prove  this  by  removing,  first, 
your  finger,  and  then  the  glass  tube.  The 
egg  is  charged  negatively. 

Again  ;  place  two  eggs,  E  E,  fig.  17, 


PIG.  17. 

lengthwise  on  two  dry  wine-glasses,  g  g, 
and  cause  two  of  their  ends  to  touch 
each  other,  as  at  c.  Bring  your  rubbed 
glass  rod  near  the  end  a,  and  while  it  is 
there  separate  the  eggs  by  moving  one 
glass  away  from  the  other.  Withdraw 
the  rod  and  test  both  eggs,  a  repels 
rubbed  sealing-wax,  and  b  repels  rubbed 
glass  ;  a  is  therefore  negative,  b  is  posi- 


tive. The  two  charges,  moreover,  exact- 
ly neutralize  each  other  in  the  oiectro- 
scope.  Again  l>rin«j  the  eggs  together 
and  restore  the  rubbed  tube  to  its  pla«o 
near  a.  Touch  a  and  then  separate .  tho 
eggs.  Remove  the  glass  rod  an-1  t^st  the 
eggs.  a  is  negative,  b  is  reutral.  Its 
electricity  lias  escaped  through  the  lin- 
ger, though  placed  ;.t  a. 

Equally  good,  if  not  indeed  more 
handy,  for  theso  experiments  arc  two 
apples  A  A,  fig.  10,  supported  on  stems 
cf  sealing-wax.  A  needle  is  healed  L;.  \ 
sni-k  in  each  case  into  the  rtick  of  wax  at 
the  top,  and  on  to  the  nccilc  the  apple  is 
pushed.  The  sealing-wax  stems  are  stuck 
on  by  melting  to  little  foot-boards.  By 
arrangements  of  this  kind  you  rnako  ex- 
periments which  are  more  instructive  than 
those  usually  made  with  instruments 
twenty  times  more  expensive. 

Push  vour  researches  still  farther,  and 
instead  of  bringing  the  eggs  or  apples  to- 
gether place  them  six  feet  or  so  apart, 


FIG.  18. 


and  let  a  light  chain,  c,  fig.  19,  or  a  wire, 
stretch  from  one  to  the  other.  Two 
brass  balls,  or  wooden  balls  covered  with 


tin-foil,  supported  by  tall  drinking  glasses, 
'    will  be  better  than  the  eggs  for  this 


GG 


experiment,  for  they  will  bear  better  the 
strain  of  the  chain  ;  but  you  can  make 
the  experiment  with  the  eggs,  or  very 
readily  with  the  two  apples  or  two  tur- 
nips. For  the  present  we  will  suppose 
the  straw-index  1  1'  not  to  be  there.  Rub 


LESSORS   IN   ELECTRICITY. 


305 


FIG.  19. 


yonr  glass  tube  B,  and  bring  it  near  one 
of  the  balls  ;  test  both  :  the  near  one,  T', 
is  negative,  the  distant  one,  T,  positive. 
Touch  the  near  one,  the  positive  elec- 
tricity, which  had  been  driven  along  the 
chain  to  the  remotest  part  of  the  system, 
returns  along  the  chnin,  passes  through 
(he  negative,  which  is  held  captive  by 
the  tube,  and  escapes  to  the  earth. 
When  the  tube  R  is  removed,  negative 
electricity  overspreads  both  chain  and 
balls. 

In  fig.  8  you  made  the  acquaintance  of 
the  plate  N,  and  the  straw-index  i  iy, 
shown  on  a  smaller  scale  in  fig.  19.  By 
their  means  you  immediately  see  both 
the  effect  of  the  first  induction,  and  the 
consequence  of  touching  any  part  of  tha 
system  with  the  finger.  The  plate  K 
rests  over  the  ball  or  turnip  T,  the  posi- 
tion of  the  straw-index  being  that  shown 
by  the  dots.  Bring  the  rubbed  tube  neat 
T'  :  the  end  N  of  the  index  immediately 
descends  and  the  other  end  rises  along 
the  graduated  scale.  Remove  the  glass 
rod;  the  index  1 1'  immediately  falls. 
Practice  this  approach  and  withdrawal, 
and  observe  how  promptly  the  index  de- 
clares the  separation  and  rccomposition 
of  the  fluids. 

While  the  tube  is  near  T',  and  the  end 
ST  of  the  index  is  attracted,  let  T'  be 
touched  by  the  finger.  The  end  N  is  im- 
mediately liberated,  for  the  electricity 
which  pulled  it  down  escapes  along  the 
chain  and  through  the  finger  to  the 
earth.  Now  remove  your  excited  tube. 
The  captive  negative  electricity  diffuses 
itself  over  both  balls,  arid  the  index  is 
again  attracted. 

Instead  of  the  chain  you  may  interpose 
between  the  balls  100  feet  of  wire  sup- 


by silk  loops.  This  is  done  m 
fig.  20,  which  shows  the  wire  w  support- 
ed by  the  silk  strings  8  s  s.  For  the  bafl 
or  turnip  T',  fig.  19,  the  cylinder  c,  on  a 
glass  support  a,  is  substituted,  the  little 
table  M  taking  the  place  of  the  ball  T. 
Every  approach  and  withdrawal  of  the 
rubbed  glass  tube  R  is  followed  obedient- 
ly by  the  attraction  and  liberation  of  N, 
and  the  corresponding  motion  of  the  in- 
dex N  I. 

Repeat  here  an  experiment,  first 


306 


LESSONS  IN   ELECTRICITY. 


by  a  great  electrician  named  ^Epinns.  I 
wish  you  to  make  these  historic  experi- 
ments. Insulate  an  elongated  metal  con- 
ductor, c  c',  fig.  21,  or  one  formed  of 
wood  coated  with  tin-foil — even  a  carrot, 
cucumber,  or  parsnip,  so  that  it  be  insu- 
lated, will  answer.  Let  a  small  weight, 
W,  suspended  from  a  silk  string,  s,  re^t 
en  one  end  of  the  conductor,  and  hold 


FLO.  2L 

your  rubbed  glass  tube,  R,  over  the  other 
end.  You  can  predict  beforehand  what 
will  occur  when  you  remove  the  weight. 
It  carries  away  with  it  electricity,  which 
repels  rubbed  glass,  and  attracts  your 
balanced  lath. 

Stand  on  an  insulating  stool  ;  or  make 
one  by  placing  a  board  on  four  warm 
tumblers.  Present  the  knuckles  of  your 
riirht  hand  to  the  end  of  tho  balanced 
lath,  and  stretch  forth  your  left  arm. 
There  is  no  attraction.  But  let  n  friend 
or  an  assistant  bring  the  rubbed  glass 
tube  over  the  left  arm  ;  the  kUi  immedi- 
ately follows  the  right  Land. 

Touch  the  lath,  or  any  (.th'-r  uninsulat- 
ed body  ;  the  **  attractive  virtue,"  as  it 
was  called  by  Gray,  disappears.  After 
this,  as  long  r.&  the  "excited  tul»  is  held 


over  the  arm  there  in  no  attraction.  But 
when  the  tube  is  removed  the  attractive 
power  of  the  hand  is  restored.  Here  the 
first  attraction  was  by  positive  -electricity 
driven  to  the  right  Land  from  the  left, 
and  the  second  attraction  by  negative 
electricity,  liberated  by  the  removal  of 
the  glass  rod.  Experiment  proves  the 
logic  of  theory  to  be  without  a  flaw. 

Stand  on  an  insulating  stool,  and  place 
your  right  hand  on  the  electroscope  ; 
there  is  no  action.  Stretch  forth  the  left 
arm  and  permit  an  assistant  alternately 
to  bring  near,  and  to  withdraw,  an  excit- 
ed glass  tube.  The  gold  leaves  open  ?;nd 
collapse  i.i  similar  alternation.  At  every 
approach,  positive  electricity  is  driven 
over  the  gold  leaves  ;  at  every  with- 
drawal, the  equilibrium  is  restored. 

We  arc  now  in  a  condition  to  repeat, 
with  case,  the  experiment  of  Du  Fay- 
mentioned  in  §  1.°,.  A  board  is  support- 
ed by  four  silk  ropes,  and  on  the  board 
is  stretched  a  boy.  Bring  his  fore  Lend, 
or  better  still  his  nose,  under  the  end  of 
your  straw-index  1 i',  fig.  22.  Then 
bring  down  over  his  legs  your  rulbed 
glass  tube  ;  instantly  the  end  i'  is  attzact- 
ed  and  the  end  i  rises  along  the  graduat- 
ed scale.  Before  the  end  i'  comes  into 
contact  with  the  nose  or  forehead  a  spark 
passes  between  it  and  the  boy. 


LESSONS  IN  ELECTRICITY. 


307 


1  will  now  ask  you  to  charge  your 
Dutch  metal  electroscope  (fig.  7)  posi- 
tively by  rubbed  gutta-percha,  and  to 
charge  it  negatively7  by  rubbed  glass.  A 
moment's  reflection  will  enable  you  to  do 
it.  You  bring  your  excited  bodv  near  : 
the  same  electricity  as  that  of  the  excited 
body  is  driven  over  the  leaves,  and  they 
diverge  by  repulsion.  Touch  the  elec- 
troscope, the  leaves  collapse.  Withdraw 
your  finger,  and  withdraw  afterwards  the 
excited  body  :  the  leaves  then  diverge 
with  the  opposite  electricity. 

The  simplest  way  of  testing  the  quality 
of  electricity  is  to  charge  the  electroscope 
with  electricity  of  a  known  kind.  If,  on 
the  approach  of  the  body  to  be  tested, 
the  leaves  diverge  still  wider,  Ilio  l™vr<* 
and  the  body  are  similarly  electrified. 
The  reason  is  obvious. 

Omitting  the  last  experiment,  the 
wealth  of  knowledge  which  these  re- 
searches involve  might  be  placed  within 
any  intelligent  boy's  reach  by  the  wise 
expenditure  of  half-a-crovvn. 

Once  firmly  possessed  of  the  principle 
of  induction  and  versed  in  its  application, 
the  difficulties  of  our  subject  will  melt 
away  before  us.  In  fact  oar  subsequent 
work  will  consist  mainly  in  unravelling 
phenomena  by  the  aid  of  this  principle. 

Without  a  knowledge  of  this  principle 
wo  could  render  no  account  of  the  attrac- 
tion of  neutral  bodies  by  our  excited 
tubes.  In  reality  the  attracted  bodies 
arc  not  neutral  :  they  are  first  electrified 
by  influence,  and  it  is  because  they  are 
thus  electrified  that  they  are  attracted. 

This  is  the  place  to  refer  more  fully 
to  a  point  already  alluded  to.  Neutral 
bodies,  as  just  stated,  arc  attracted,  be- 
cause they  arc  really  converted  into  elec- 
trified bodies  by  induction.  Suppose  a 
body  to  be  feebly  electrified  positively, 
and  that  you  bring  your  rubbed  glass 
tubo  t:>  bear  upon  the  body.  You  clear- 
ly sec  that  the  induced  negative  electricity 
may  be  strong  enough  to  mask  and  over- 
come the  weak  positive  charge  possessed 
by  the  body.  We  should  thus  have  two 
bodies  electrified  alike,  attracting  each 
other.  This  is  the  danger  against  which 
I  promised  to  warn  you  in  §  10,  where 
the  test  of  attraction  was  rejected. 

Wo  will  now  apply  the  principle  of  in- 
duction to  explain  a  very  beautiful  inven- 
tion, made  known  by  the  celebrated 


Voltain  1775. 

§  15.    The  Mectrophorus. 

Cut  a  circle,  T,  fig.  23,  6  inches  in 
diameter  out  of  sheet  zinc,  or  out  of 
common  tin.  Heat  it  at  its  centre  by  the 


FIG.  23. 

flame  of  a  spirit-lamp  or  of  a  candle. 
Attach  to  it  there  a  stick  of  sealing-wax, 
H,  which,  when  the  metal  cools,  i*  to 
serve  as  an  insulating  handle. — You  have 
now  the  lid  of  the  electrophorus.  A 
resinous  surface,  or  what  is  simpler  a 
sheet  of  vulcanized  india-rubber,  p,  or 
even  of  hot  brown  paper,  will  answer  for 
the  plate  of  the  electrophorus. 

Rub  your  "  plate"  with  flannel,  or 
whisk  it  briskly  with  a  fox's  brush.  It 
is  thereby  negatively  electrified.  Place 
thy  "  lid  "  of  your  olectrophorus  on  the 
excited  surface  :  it  touches  it  at  a  few 
points  only.  For  the  most  pait  lid  and 
plate  arc  separated  by  a  film  of  air. 

The  excited  surface  acts  by  induction 
across  this  film  upon  the  lid,  attracting 
its  positive  and  repelling  its  negative 
electricity.  You  Lnvo  in  fact  in  the  lid 
two  layers  of  electricity,  the  lower  one, 
which  is  "  bound,"  positive  ;  tho  upper 
one,  which  is  "  free,"  negative.  ^Lift 
the  lid  :  the  electricities  iiow  again  to- 
gether ;  neutrality  is  restored,  and  your 
lid  fails  to  attract  your  balanced  lath. 

Once  more  plnce  the  lid  upon  the  ex- 
cited surface  :  touch  it  with  the  finger. 
What  occurs  ?  You  ought  to  know. 
Tho  free  electricity, ,  which  is  negative, 
will  escape  through  your  body  to  tho 
earth,  leaving  the  chained  positive  be- 
hind. 

Now  lift  the  lid  by  the  handle  :  what 
is  its  condition  ?  Again  I  say  you  ought 


LESSORS  IN  ELECTRICITY. 


Fw.  24. 


to  know.  It  is  covered  with  free  posi- 
tive electricity.  If  it  be  presented  to  the 
lath  it  will  strongly  attract  it  :  if  it  be 
presented  to  the  knuckle  it  will  yield  a 
spark. 

A  iniooth  half-crown,  or  a  penny,  will 
answer  for  this  experiment.  Stick  to  tho 
coin  an  inch  of  sealing-wax  as  an  insulat- 
ing handle  :  bring  it  down  upon  the  ex- 
cited india-rubber  :  touch  it,  lift  it,  and 
present  it  to  your  lath.  The  lath  may 
be  six  or  eight  feet  long,  three  inches 
wide  and  half  an  inch  thick  ;  the  little 
electrophorus  lid,  formed  by  the  half 
crown,  will  pull  it  round  and  round.  The 
experiment  is  a  very  impressive  one. 

Scrutinize  your  instrument  still  further. 
Let  the  end  of  a  thin  wire  rest  upon  the 
lid  of  your  electrophorus,  under  a  little 
weight  if  necessary  ;  and  connect  the  other 
end  of  the  wire  with  the  electroscope. 
As  you  lower  the  lid  down  toward  the 
excited  plate  of  the  electrophorus,  what 
must  occur  ?  The  power  of  prevision 
now  belongs  to  you  and  you  must  exer- 
cise it.  The  repelled  electricity  will  flow 
over  the  leaves  of  the  electroscope,  caus- 
ing them  to  diverge.  Lift  the  lid,  they 
oollapse.  Lower  and  raise  the  lid  several 
times,  and  observe  the  corresponding 
rhythmic  action  of  the  electroscope  leaves. 

A  little  knob  of  sealing-wax,  B,  coated 
with  tin-foil,  or  indeed  any  knob  with  a 
conducting  surface,  stuck  to  the  lid  of 
the  electrophorus,  will  enable  you  to  ob- 
tain a  better  spark.  The  reason  of  this 
will  immediately  appear. 

More  than  half  the  ralne  of  your  pres- 
ent labor  consists  in  arranging  each  ex- 
periment in  thought  before  it  is  realized 
in  fact  ;  and  more  than  half  the  delight 
of  your  work  will  consist  in  observing 
the  verification  of  what  you  have  foreseen 
and  predicted. 

§  16.  Action  of  Points  and  Flames. 


The  course  of  exposition  proceeds 
naturally  from  the  electrophorus  to  the 
electrical  machine.  But  before  we  take 
up  the  machine  we  must  make  our  minds 
clear  regarding  the  manner  in  which  elec- 
tricity diffuses  itself  over  conductors,  and 
more  especially  over  elongated  and  point- 
ed conductors. 

Rub  your  glass  tube  and  draw  it  over 
an  insulated  sphere  of  metal — of  wood 
covered  with  tin-foil,  or  indeed  any  other 
insulated  spherical  conductor.  Repeat 
the  process  several  times,  so  as  to  impart 
a  good  charge  to  the  sphere.  Touch  the 
charged  sphere  with  your  carrier,  and 
transfer  the  charge  to  the  electroscope. 
Note  the  divergence  of  the  leaves.  Dis- 
charge the  electroscope,  and  repeat  tho 
experiment,  touching,  IIOWCVPV,  some 
other  point  of  the  sphere.  The  electro- 
scope shows  sensibly  the  same  amount  of 
divergence.  Even  when  the  greatest  ex- 
actness of  the  most  practised  experi- 
menter is  brought  into  play,  the  spherical 
conductor  is  found  to  be  equally  charged 
at  all  points  of  its  surface.  You  may 
figure  the  electric  fluid  as  a  little  ocean 
encompassing  the  sphere,  and  of  the  same 
depth  everywhere. 

But  supposing  the  conductor,  instead 
of  being  a  sphere,  to  be  a  cube,  an  elon- 
gated cylinder,  a  cone,  or  a  disk.  The 
depth,  or  as  it  is  sometimes  called  the 
density,  of  the  electricity,  will  not  be 
everywhere  the  same.  The  corners  of 
the  cube  will  impart  a  stronger  charge  to 
your  carrier  than  the  sides.  The  end  of 
the  cylinder  will  impart  a  stronger  charge 
than  its  middle.  The  edge  of  the  disk 
will  impart  a  stronger  chargo  than  its  flat 
surface.  The  apex  or  point  of  the  cone 
will  impart  a  stronger  chargo  than  its 
curved  surface  or  i:s  base. 

You  can  satisfy  yourself  of  the  truth  of  all 
this  in  a  rough,  but  certain  way,  by  charg- 
ing, after  the  sphere,  a  turnip  cut  into  the 


LESSOST3  m  ELECTRICITY. 


form  of  a  cube  ;  or  a  cigar-box  coated  with 
tin-foil  ;  a  metal  cylinder,  or  a  wooden 
one  coated  with  tin-foil  ;  a  disk  of  tin  or 
of  sheet  zinc  ;  a  carrot  or  parsnip  with  its 
natural  shape  improved  so  as  to  make  it 
a  sharp  cone.  You  will  find  the  charge 
Imparted  to  the  carrier  by  the  sharp  cor- 
ners and  points  of  such  bodies,  when  elec- 
trified, to  be  greater  than  that  communi- 
cated by  the  gently  rounded  or  flat  sur- 
faces. The  difference  may  not  be  great, 
but  it  will  be  distinct.  Indeed  an  egg 
laid  on  its  side,  as  we  have  already  used 
it  in  our  experiments  on  induction  (fig. 
1C),  yields  a  stronger  charge  from  its 
ends  than  from  its  middle. 


Fia.  25. 

Let  me  place  before  you  an  example  of 
this  distribution,  taken  from  the  excellent 
work  on  "  Frictional  Electricity"  by  Pro- 
fessor Riess  of  Berlin.  Two  cones,  fig. 
24,  are  placed  together  base  to  base. 
Calling  the  strength  of  the  charge  along 
the  circular  edge  where  the  two  bases  join 
eacli  other  100,  the  charge  at  the  apex  of 
the  blunter  cone  is  133  ;  and  at  the  apex 
of  the  sharper  one  202.  The  other  num- 
bers give  the  charges  taken  from  the 
points  where  they  are  placed.  Fig.  25, 
moreover,  represents  a  cube  with  a  cone 
placed  upon  it.  The  charge  on  the  faco 
of  the  cube  being  1,  the  charges  at  fho 
corners  of  the  cube  and  at  the  apex  of 
the  cone  are  given  by  the  other  numbers; 
they  are  all  far  in  excess  of  the  electricity 
on  the  flat  surface. 

Iliess  found  that  he  fould  deduce  with 
great  accuracy  the  sharpness  of  a  point, 
from  the  charge  which  it  imparted,  lie 
compared  iu  this  way  the  sharpness  of 
various  thoins,  with  that  of  a  firio  Knolish 
Rcwing'needle.  The  following  is  the  re- 
sult : — -Euphorbia  thorn  was  si  mi  per  than 
the  needle  ;  gooseberry  thorn  of  ilu  same 


sharpness  as  the  needle  ;  while  cactus, 
blackthorn,  and  rose,  fell  more  and  more 
behind  the  needle  in  sharpness.  Calling, 
i^r  example,  the  charge  obtained  from 
c  iphorbia  90  ;  that  obtained  from  the 
needle  was  80,  and  from  the  rose  only 
53. 

Considering  that  each  electricity  is 
self-repulsive,  and  that  it  heaps  itself  up 
upon  a  point,  in  the  manner  here  shown, 
you  will  have  little  difficulty  in  conceiv- 
ing that  when  the  charge  of  a  conductor 
carrying  a  point  is  sufficiently  strong,  the 
electricity  will  finally  disperse  itself  by 
streaming  from  the  point. 

The  following  experiments  are  theoret- 
ically important  :  Attach  a  stick  of 
sealing-wax  to  a  small  plate  of  tin  or  of 
wood,  so  that  the  stick  may  stand 
upright.  Heat  a  needle  and  insert  it  into 
the  top  of  the  stick  of  wax  ;  on  this 
needle  mount  horizontally  a  carrot.  You 
have  thus  an  insulated  conductor.  Stick 
into  your  carrot  at  one  of  its  ends  a  sew- 
ing needle  ;  and  hold  for  an  instant  your 
rubbed  glass  tube  in  front  of  this  needle 
without  touching  it.  AVhat  occurs?  The 
negative  electricity  of  the  carrot  is  imme- 
diately discharged  from  the  point  against 
the  glass  tube.  Remove  the  tube,  test 
the  carrot  :  it  is  positively  electrified. 

And  now  for  another  experiment,  not 
BO  easily  made,  but  still  certain  to  succeed 
if  you  are  careful.  Excite  your  glass  rod, 
turn  your  needle  away  from  it,  and  bring 
the  rod  near  the  other  end  of  the  carrot. 
What  occurs  ?  The  positive  electricity 
is  now  icpe'llcd  to  the  point,  from  which 
it  will  stream  into  the  air.  Remove  the 
rod  and  test  the  carrot  :  it  is  negatively 
electrified. 

Again  turn  the  point  toward  you,  and 
place  in  front  of  it  a  plate  of  dry  glass, 
wax,  resin,  shellac,  paraffin,  gutta-percha 
or  any  other  insulator.  Pass  your  rubbed 
glass  tube  once  downwards  or  upwards, 
the  insulating  plate  being  between  the  ex- 
cited tabe  and  the  point.  The  point  will 
discharge  its  electricity  against  the  insu- 
lating plate,  which  on  trial  will  be  found 
negatively  electrified. 


§  17.    The  Electrical  Machine. 

An  electrical  machine  consists  of  two 
principal    parts  :  the  insulator  which  is 


SiO 


LESSOXS  IN  ELECTRICITY. 


PIG.  26. 

excited  by  friction,  and  the  * '  prime  con 
ductor." 

The  sulphur  sphere  of  Otto  von  Guc- 
rickc  was,  as  already  stated,  the  first 
electrical  machine.  The  hand  was  the 
rubber,  and  indeed  it  long  continued  to 
be  so.  For  the  sulphur  sphere,  Ilauks- 
bcc  and  Vv'incLlcT  substituted  globes  cf 
glass.  Doze  of  Wittenberg  (1741)  add- 
ed the  prime  conductor,  which  was  at  f:r  t 
a  tin  tube  supported  by  resin,  orouspcnu- 
ed  by  sill:.  Soon  afterward  Gordon 
substituted  a  glass  cylinder  for  the  globe. 
It  "was  sometimes  mounted  vertically, 
sometimes  horizontally.  Gordon  so  in- 
tensified l.i.'i  discharges  as  to  be  able  to 
kill  email  birds  with  them.  In  17CO 
Planta  introduced  the  plate  machine  now 
commonly  in  r.se. 

Mr.  Cottrell  has  constructed  for  these 
Lessons  the  email  cylinder  machine  shown 
in  fig.  20.  The  glass  cylinder  is  about  7 
inches  long  and  4  inches  in  diameter  ; 
its  cost  is  eighteen  pence.  Through  the 
cylinder  passes  tightly,  as  an  axis,  a 
pidcc  of  lath,  rendered  secure  by  sealing- 
wax  where  it  enters  and  where  it  quits 
the  cylinder.  G  is  a  glass  rod  supporting 
the  conductor  c,  which  is  a  piece  of  lath 
coated  with  tin-foil.  Into  the  lath  is 
driven  the  scries  of  pin  points,  r,  r.  The 
rubber  n,  is  cccn  at  the  further  side  cf 
the  cylinder,  supported  by  the  upright 
lath  r/,  and  caused  to  press  against  the 
glass.  G'  ha  flap  of  u'lk  attached  to  the 
rubber.  Y/hen  the  handle  13  turned 


FIG.  27. 

sparks  may  be  taken,  or  a  Leyden  jar 
charged  at  the  knob  c. 

A  plato  machine  h  thown  i:i  fig.  27. 
p  b  the  plate,  which  t;:rn3  on  an  axi.i 
passing  through  its  centre  ;  r.  and  r/  arc 
two  rubbers  which  clasp  tho  plate,  with 
the  flips  of  cilk  G  G'  attached  to  them.  A 
and  A'  arc  rows  of  points  forming  part  cf 
the  prime  conductor,  c.  G  G'  is  an  insu- 
lating rod  cf  glass,  which  cuts  off  tho 
connection  between  the  conductor  and 
the  handle  of  the  machine. 

The  prime  conductor  is  charged  in  the 
following  manner.  When  the  glass  plato 
is  turned,  as  it  passes  each  rubber  it  is 
positively  electrified.  Facing  tho  elec- 
trified glass  is  the  row  of  points,  placed 
midway  between  the  two  rubbers.  On 
these  points  the  glass  acts  by  induction, 
attracting  the  negative  and  repelling  the 
positive.  In  accordance  with  the  princi- 
ples already  explained  in  §10,  tho  nega- 
tive electricity  streams  from  tho  points 
against  the  excited  glass,  which  then 
passes  on  neutralized  to  the  next  rubber, 
where  it  is  again  excited. 

Thus  the  prime  conductor  is  charged, 
not  by  tho  direct  communication  to  it  of 
positive  electricity,  but  by  depriving  it 
of  its  negative. 

If  when  tho  conductor  is  charged  you 
bring  tho  knuckle  near  it,  the  electricity 
passes  from  the  conductor  to  tho  knuckle 
in  the  form  of  a  spark. 

Take  this  spark  with  the  blunt  knuckle 
while  the  machine  is  being  turned  ;  and 


LESSON'S  IN  "ELECTRICITY. 


311 


FIG.  28. 

then  try  the  effect  of  presenting  the  finger 
ends,  instead  of  the  knuckle,  to  the  con- 
ductor. The  spark  falls  exceedingly  in 
brilliancy.  Substitute  for  the  finger  ends 
a  needle  point  :  you  fail  to  get  a  spark 
at  nil.  To  obtain  a  good  spark  the  elec- 
tricity upon  the  prime  conductor  must 
reach  a  sufficient  density  (or  tension  as  it 
i*  sometimes  called)  ;  and  to  secure  this 
no  points  from  which  thu  electricity  can 
stream  out  must  exist  on  the  conductor, 
or  be  presented  to  it.  All  parts  of  tho 
conductor  are  therefore  carefully  rounded 
off,  sharp  points  and  edges  being  avoided. 
It  u  usual  to  attach  to  the  conductor 
an  electroscope  consisting  of  an  upright 
metal  stem,  A  c,  fig.  28,  to  which  a  straw 
with  a  pith  ball,  u,  at  its  free  end,  is  at- 
tached. The  straw  turns  loosely  upon  a 
pivot  at  c.  The  electricity  passing  from 
the  conductor  is  diffused  over  the  whole 
electroscope,  and  the  straw  and  stem  be- 
ing both  positively  electrified,  repel  each 
other.  The  straw,  being  the  movable 
body,  flies  away.  The  amount  of  the 
divergence  is  measured  upon  a  graduated 
arc. 

§  18,  Further  Experiments  on  the  Action 
of  Points.  — The  Electric  Mill.  — The 
Golden  Fish. — Lightning  Conductors. 

If  no  point  exist  on  the  conductor,  Q 
single  turn  of  the  handle  of  the  machine 
usually  suffices  to  cause  the  straw  to  stand 
out  at  a  large  angle  to  the  stem.  If,  on 
the  contrary,  a  point  be  attached  to  the 
conductor,  you  cannot  produce  a  large 
divergence,  because  the  electricity,  as 


FIG.  29. 

fast  as  it  is  generated,  is  dispersed  by  the 
point.  The  same  effect  is  observed  when 
you  present  a  point  to  the  conductor. 
The  conductor  acts  by  induction  upon  the 
point,  causing  the  negative  electricity  to 
stream  from  it  against  the  conductor, 
which  is  thus  neutralized  almost  as  fast  as 
it  is  charged.  Flames  and  glowing  em- 
bers act  like  points  ;  they  also  rapidly 
discharge  clectiicity. 

The  electricity  escaping  from  a  point 
or  flame  into  the  air  renders  the  air  self 
repulsive.  The  consequence  is  that  when 
the  hand  is  placed  over  a  point  mounted 
on  the  prime  conductor  of  a  machine  in 
pood  action,  a  cold  blast  is  distinctly  felt. 
Dr.  Watson  noticed  this  blast  from  n 
flame  placed  on  an  electrified  conductor  ; 
while  Wilson  noticed  the  blast  from  a 
point.  Jallabert  'and  the  Abbe  Nollet 
also  observed  and  described  the  influence 
of  points  and  flames.  The  blast  is  called 
the  "  electric  wind."  Wilson  moved 
bodies  by  its  action  :  Faraday  caused  it 
to  depress  the  surface  of  a  liquid  :  Ham- 
ilton employed  the  reaction  of  the  electric 
wind  to  make  pointed  wires  rotate.  The 
"  wind  "  was  also  found  to  promote 
evaporation. 

Hamilton's  apparatus  is  called  the 
"  electric  mill."  Make  one  for  yourself 
thus  :  Place  two  straws  s  s',  88%  fig.  29, 
about  eight  inches  long,  across  each  other 
ht  a  right  angle.  Stick  them  together  at 
their  centres  by  a  bit  of  sealing-wax. 
Pass  a  fine  wire  through  each  straw,  and 
bend  it  where  it  issues  from  the  straw, 
so  as  to  form  a  little  pointed  arm  perpea- 


312 


LESSONS  IN  ELECTRICITY. 


FIG.  30. 


dicnlar  to  the  straw,  and  from  half  an 
inch  to  three  quarters  of  an  incli  long. 
It  is  easy,  by  means  of  a  bit  of  cork  or 
sealing-wax,  to.  fix  the  wire  so  that  the 
little  bent  arms  shall  point  not  upward  or 
downward,  but  sideways,  when  the  cross 
is  horizontal.  The  points  of  sewing 
needles  may  also  be  employed  for  the  bent 
arms.  A  little  bit  of  straw  stuck  into  the 
cross  at  the  centre  forms  a  cap.  This 
slips  over  a  sewing  needle,  N,  supported 
by  a  stick  of  sealing-wax,  A.  Connect 
the  sewing  needle  with  the  electric 
machine,  and  turn.  A  wind  of  a  certain 
force  is  discharged  from  every  point,  and 
the  cross  is  urged  round  with  the  same 
force  in  the  opposite  direction. 


Place  your  left  hand  on  the  prime  con- 
ductor of  your  machine.     Let  the  hand)® 


You 


of     course,     so 


arrange  the  points  that  the  wind  from 
some  of  them  would  neutralize  the  wind 
from  others.  But  the  little  pointed  arms 
are  lo  be  so  bent  that  the  reaction  in 
every  case  shall  not  oppose  but  add  itself 
to  the  others. 

The  following  experiments  will  yield 
you  important  information  regarding  the 
action  of  points.  Stand,  as  you  have  so 
often  done  before,  upon  a  board  support- 
ed by  four  warm  tumblers.  Hold  a  small 
sewing  needle,  with  its  point  defended 
by  the  forefinger  of  your  right  hand, 
toward  your  Dutch  metal  electroscope. 


31. 


be  turned  by  a  friend  or  an  assistant  :  tho 
leaves  of  the  electroscope  open  out  a  lit- 
tle. Uncover  the  needle  point  by  the  re- 
moval of  your  finger  ;  the  leaves  at  once 
fly  violently  apart. 

Mount  a  stout  wire  upright  on  the  con- 
ductor, c,  fig.  30,  of  your  machine  ;  or 
sapport  the  wire  by  sealing-wax,  gutta- 


LESSONS  IK  ELECTRICITY. 


813 


FIG.  32. 


Fio.  33. 


FIQ.  34. 


pcrcha  or  glass,  at  a  distance  from  the 
conductor,  and  connect  both  by  a  fmo 
wire.  Bend  your  stout  wire  into  a  hook, 
and  hang  from  it  a  tassel,  T,  composed 
of  many  strips  of  light  tissue  paper. 
Work  the  machine.  Electricity  from  the 
conductor  flows  over  the  tassel,  and  the 
strips  diverge.  Hold  your  closed  fist 
toward  the  tassel,  the  strips  of  paper 
stretch  toward  it.  Hold  the  needle,  de- 
fended by  the  finger,  toward  the  tassel  : 
attraction  also  ensues.  Uncover  the  needle 
without  moving  the  hand  ;  the  strips 
retreat  as  if  blown  away  by  a  wind. 
Holding  the  needle,  N,  fig.  31,  upright 
underneath  the  tassel,  its  strips  discharge 
themselves  and  collapse  utterly. 

And  now  repeat  Du  Fay's  experiment 
which  led  to  the  discovery  of  two  electric- 
ities. Excite  your  glass  tube,  and  hoi  1 
it  in  readiness  while  a  friend  or  an  assist- 
ant liberates  a  real  gold  or  silver  leaf  in 
the  air.  Bring  the  tube  near  the  leaf  : 
it  plunges  toward  the  tube,  stops  sud- 
denly, and  then  flies  away.  You  may 
chase  it  round  the  room  for  hours  with- 
out permitting  it  to  reach  the  ground. 
The  leaf  is  first  acted  upon  inductively 
by  the  tube.  It  is  powerfully  attracted 


for  a  moment,  and  rusnrs  toward  the 
tube.  But  from  its  thin  edges  and  cor- 
ners the  negative  electricity  streams  forth, 
leaving  the  haf  positively  electrified. 
Repulsion  then  sets  in,  because  tube  and 
leaf  are  electrified  alike,  as  shown  in  fig. 
32.  The  retreat  of  the  tassel  in  the  last 
experiment  is  due  to  a  similar  cause. 

There  is  also  a  discharge  of  positive 
electricity  into  the  air  from  the  more  dis- 
tant portions  of  the  gold -leaf,  to  which 
that  electricity  is  repelled.  Both  dis- 
charges are  accompanied  by  an  electric 
wind.  It  is  possible  to  give  the  gold- 
leaf  a  shape  which  shall  enable  it  to  float 
securely  in  the  air,  by  the  reaction  of  the 
two  winds  issuing  from  its  opposite  ends. 
This  is  Franklin's  experiment  of  the 
Golden  Fish.  It  was  first  made  with  the 
charged  conductor  of  an  electrical  ma- 
chine. M.  Srtsczek  revived  it  in  a  more 
convenient  form,  using  instead  of  the 
conductor  the  knob  of  a  charged  Leyden 
jar.  You  may  walk  round  a  room  with 
the  jar  in  your  hand  ;  the  "  fish  "  will 
obediently  follow  in  the  air  an  inch  or 
two,  or  even  throe  inches,  from  the 
knob.  Sec  A  B,  fig.  33.  Even  a  hasty 
motion  of  the  jar  will  not  shake  it  away, 


814 


"LESSONS  tff  ELECTRICITY. 


Well  -  pointed  lightning  conductors, 
when  acted  on  by  a  thunder  cloud,  dis- 
charge their  induced  electricity  against 
the  cloud.  Franklin  raw  this  with  great 
clearness,  and  illustrated  it  with  great  in- 
genuity. The  under  side  of  a  thunder 
cloud,  when  viewed  horizontally,  he 
observed  to  be  inched,  composed,  in 
fact,  of  fragments  one  below  the  other, 
sometimes  reaching  near  the  eaith. 
These  he  regarded  «is  so  many  stepping- 
stones  Y/hich  assist  in  conducting  the 
stroke  of  the  cloud.  To  represent  those 
by  experiment  he  took  two  or  three  locks 
of  fine  loose  cotton,  tied  them  ir,  a  row, 
and  hiing  them  from  his  pr.'n.ic  con- 
ductor. When  this  was  excited  the  locks 
stretched  downward  toward  tho  earth  ; 
but  by  presenting  a  sharp  point  erect 
under  the  lowest  bunch  of  cotton,  it 
shrunk  upward  to  that  above  it,  nor  did 
the  shrinking  cease  till  all  the  locks  had 
retreated  to  the  prime  conductor  itself. 
"  May  not,"  SMVS  Franklin,  "the  small 
ek'Ctrified  cloud,  whose  equilibrium  with 
the  earth  is  so  soon  restored  by  the  point, 
rise  up  to  the  main  body,  and  by  that 
means  occasion  so  large  a  vacancy  that 
the  grand  cloud  cannot  strike  in  that 
placet" 

§  19.  History  of  the  Ley  den  Jar. — The 
Ley  den  Battery. 

The  next  discovery  which  we  have  to 
master  throws  all  former  ones  into  the 
shade.  It  was  first  announced  in  a  letter  ad- 
dressed on  the  4th  of  November,  1V45, 
to  Dr.  Liebcrkuhn,  of  Berlin,  by  Kleist, 
a  clergyman  of  Cammin,  in  Fomerania. 
By  means  of  a  cork,  c,  fig.  34,  he  fixed  a 
r.ail,  N,  in  a  phial,  G,  into  which  he  had 
poured  a  little  mercury,  spirits,  or  water, 
w.  On  electrifying  tho  nail  he  was  ablo 
to  pass  from  one  room  into  another  wit: 
the  phial  in  his  hand  «nd  to  ignite  spirits 
of  wine  with  it.  "  If,"  said  he,  *4  while 
it  is  electrifying  I  put  my  finger,  or  a 
piece  of  gold  which  I  hold  in  my  hand, 
to  the  nail,  I  receive  a  shock  which  stuns 
my  arms  and  shoulders." 

In  the  following  year  Cunaeus  of  Ley- 
den  made  substantially  the  ssme  discov- 
ery. It  caused  great  wonder  #nd  dread, 
which  arose  chietly  from  the  excited  im- 
agination. Musschcnbroekfclt  the  shock, 
and  declared  in  a  letter  to  a  friend  that 


lie  would  not  take  a  second  one  for  th« 
crown  of  France.  Bleeding  at  the  nose, 
ardent  fever,  a  heaviness  of  head  which 
endured  for  days,  were  all  ascribed  to  the 
shock.  Boze  wished  that  he  might  die 
of  it,  so  that  he  might  enjoy  the  honor 
of  having  his  death  chronicled  in  the 
Paris  "  Academy  of  Sciences."  Kleist 
missed  the  explanation  of  the  phenome- 
non ;  while  the  Leyden  philosophers  cor- 
rectly stated  the  conditions  necessary  to 
the  success  of  the  experiment.  Hence 
tlio  paial  received  the  name  of  the  Ley- 
den phial,  or  Leyden  jar. 

Tho  discovery  of  Kleist  and  Cunasus 
excited  the  most  profound  interest,  and 
the  subject  was  explored  in  all  directions. 
Wilson  in  174G  filled  a  phial  partially 
with  water,  and  plunged  it  into  water,  so 
as  to  bring  the  water  surfaces,  within 
and  without  the  phial,  to  the  same  level. 
On  charging  such  a  phial  the  strength  of 
the  shock  was  found  greater  than  had 
oeen  observed  before. 

Two  years  subsequently  Dr.  Watson 
And  Dr.  Bevis  noticed  how  the  charge 
grew  stronger  as  the  area  of  the  conduct- 
or in  contact  with  the  outer  surface  of 
the  phial  increased.  They  substituted 
shot  for  water  inside  the  jar,  and  ob- 
tained substantially  the  same  effect.  Dr. 
Bevis  then  coated  a  plate  of  glass  on  both 
sides  with  silver  foil,  to  within  about  an 
inch  of  the  edge,  and  obtained  from  it 
discharges  as  strong  as  those  obtained 
from  a  phial  containing  half  a  pint  of 
water.  Finally  Dr.  Watson  coated  his 
phial  inside  and  out  with  silver  foil.  By 
these  steps  the  Leyden  jar  reached  the 
form  which  it  possesses  to-day. 

It  is  easy  to  repeat  the  experiment  of 
Dr.  Bevis.  Procure  a  glass  plate  nine 
inches  square  ;  cover  it  0:1  both  sides,  as 
he  did,  with  tin-foil  seven  inches  square, 
leaving  the  rirn  uncovered.  Connect  one 
side  with  tho  earth,  and  the  other  with 
tho  machine.  Charge  and  discharge  : 
you  obtain  a  brilliant  spark. 

In  our  experiment  with  the  Golden 
Fish.  (fig.  33),  we  employed  a  common 
form  of  the  Leyden  j:',r,  only  with  the 
difference  that  to  get  to  a  cufiicient  dis- 
tance  from  tho  glass,  so  as  to  avoid  the 
attraction  of  the  fish  by  the  jaritsolf,  the 
knob  was  placed  higher  than  usual.  But 
with  a  good  flint-glass  tumbler,  a  piece  of 
tin-foil,  akd  a  bit  of  stout  wire,  you  can 


LESSORS   IX   E.ECTKICITr. 


make  a  jar  for  yourself.  Bad  glass,  re- 
member, is  not  rare.  In  fig.  35  you 
have  such  a  jar.  T  is  the  outer-,  T'  the 
inner  coating,  reaching  to  within  irn  inch 
c-f  the  edge  of  the  tumbler  G.  ^'  is  the 


FIG.  33. 


Fia.36. 


wire  fastened  below  by  wax,  and  sur- 
mounted by  a  knob,  which  may  be  of 
metal,  or  of  wax  or  wood,  coated  with 
tin-foil.  In  charging  the  jar  you  con- 
nect the  outer  coating  with  the  earth — 
»ay  with  a  gas-pipe  or  a  water-pipe — and 
present  the  knob  to  the  conductor  of 
your  machine.  A  few  turns  will  charge 
the  jar.  It  is  discharged  by  laying  one 
knob  of  a  **  discharger"  against  the 
outer  coating,  and  causing  the  other  knob 


to  approach  the  knob  of  the  jar.  Be- 
fore contact,  the  electricity  flies  from 
knob  to  knob  in  the  form  of  a  spark. 

A  "  discharger"  suited  to  our  means 
and  purposes  is  shown  in  fig.  36.  n  is  a 
stick  of  sealing-wax,  or,  better  still,  of 
ebonite  ;  w  w  a  stout  wire  bent  as  in  the 
figure,  and  ending  in  the  knobs  B  B'. 
These  may  be  of  wax  coated  with  tin- 
foil. Any  other  light  conducting  knobs 
would  of  course  answer.  The  insulating 
handle  n  protects  you  effectually  from 
the  shock. 

You  must  render  yourself  expert  in  the 
use  of  the  discharger.  The  mode  of 
using  it  is  shown  in  rig.  37. 

By  augmenting  the  size  of  a  Leydsn 
jar  we  render  it  capable  of  accepting  a. 
larger  charge  of  electricity.  But  there 
is  a  limit  to  the  size  of  a  jar.  When 
therefore,  larger  charges  are  required 
than  a  single  jar  can  furnish,  we  make 
use  of  a  number  of  jars.  In  tig.  38  nine 
of  them  are  shown.  All  their  interior 
coatings  are  united  together  by  brass 
rods,  while  all  Um  outer  coatings  rest 
upon  a  metal  suiface  in  free  communica- 
tion with  the  eaith. 

This  combination  of  Leydcn  jars  con- 
stitutes the  Leydcn  Battery,  the  effect  of 
which  is  equal  to  that  of  a  single  jar  ,f 
nine  times  the  size  of  one  of  the  jars. 

§  20.  Explanation  of  the  Leydcn  Jar. 
The  principles  of  electrical  induction 


LESSONS  IN  ELECTRICITY. 


FIG.  38. 


"with  which  you  arc  now  so  familiar  will 
enable  you  to  thoroughly  analyze  and 
understand  the  action  of  the  Leydcn  jar. 
In  charging-  the  jar  the  outer  coating  is 
connected  with  the  earth,  and  the  inner 
coating  with  the  electrical  machine.  Let 
the  machine,  as  usual,  be  of  glass  yield- 
ing positive  electricity.  When  it  is 
worked  the  electricity  poured  into  the  jar 
acts  inductively  across  the  glass  upon  the 
outer  coating,  attracting  its  negative  and 
repelling  its  positive  to  the  earth.  Two 
mutually  attractive  electric  layers  are  thus 
in  presence  of  each  other,  being  separa- 
ted merely  by  the  glass.  When  the  ma- 
chine is  in  good  order  and  the  glass  of 
the  jar  is  thin,  the  attraction  may  be  ren- 
dered strong  enough  to  perforate  the  jar. 
By  means  of  the  discharger  the  opposite 
electricities  are  enabled  to  unite  in  the 
form  of  a  F  par  If. 

Franklin  saw  and  announced  with  clear- 
ness the  escape  of  the  electricity  from 
the  outer  coating  of  the  jar.  His  state- 
ment is  that  whatever  be  the  quantity  of 
the  "  electric  fire"  thrown  into  the  jar, 
an  equal  quantity  was  dislodged  from  the 
outside.  We  have  now  to  prove  by  ac- 
tual experiment  that  this  explanation  is 
correct. 

Place  your  Leydcn  jar  upon  a,  table, 
and  connect  the  outer  coating  with  your 
electroscope.  There  is  no  divergence  of 
the  leaves  when  electricity  is  poured  into 
the  jar. 

But  here  the  outer  coating  is  connect- 


ed through  the  table  with  the  earth.  Let 
us  cut  off  this  communication  by  an  in- 
sulator. Place  the  jar  upon  a  board  «up- 
portcd  by  warm  tumblers,  or  upon  a 
piece  of  vulcanized  india-rubber  cloth, 
and  again  connect  the  outer  coating  with 
the  electroscope.  The  moment  electric- 
ity is  communicated  to  the  knob  of  the 
jar  the  leaves  of  Dutch  metal  diverge. 
Detach  the  wire  by  your  discharger  and 
test  the  quality  of  the  electricity — it  is 
positive,  as  theory  declares  it  must  be. 

Consider  now  the  experiment  of  Klcist 
and  Cumcus  (fig.  04).  You  will,  I  doubt 
not,  penetrate  its  meaning.  You  will  see 
that  in  their  case  the  liand  formed  the 
outer  coating  of  the  jar.  When  elec- 
tricity was  communicated  through  the 
nail  to  the  water  within,  that  electricity 
acted  across  the  glass  inductively  upon 
the  hand,  attracting  the  ono  fluid  and 
repelling  the  other  to  the  earth. 

Again,  I  say,  prove  all  things  ;  and 
what  is  here  affirmed  may  be  proved  by 
the  following  beautiful  and  conclusive  ex- 
periment :  Stand  on  your  board,  i  i'  fig. 
39,  insulated  by  its  four  tumblers  ;  or 
upon  a  sheet  of  gutta-percha,  or  vulcan- 
ized india-rubber.  Seize  the  old  Leyden 
phial,  j,  with  your  left  hand,  and  pre- 
sent the  knuckle  of  your  right  hand  to 
your  balanced  lath,  L'  L.  When  electric- 
ity is  communicated  to  the  nail,  the  lath 
i*  immediately  attracted  by  the  knuckle. 
Or  touch  your  electroscope  with  your 
right  hand  ;  when  the  phial  is  charged 


LESSORS  IN  ELECTRICITY. 


PICK  39. 


the  leaves  immediately  diverge,  by  the 
electricity  driven  from  your  left  hand  to 
the  electroscope. 

Here  the  nail  may  be  electrified  either 
by  connecting  it  with  the  prime  conduct- 
or oft  he  machine,  or  by  rubbing  it  with 
an  excited  glass  rod.  Indeed,  I  should 
prefer  your  resorting  to  the  simplest  and 
cheapest  means  in  making  these  experi- 
ments. 

§  21.   Franklin's   Cascade  Battery. 

As  a  thoughtful  and  reflective  boy  or 
girl  you  cannot,  I  think,  help  wondering 
at  the  power  which  your  thorough  mas- 
tery of  the  principles  of  induction  gives 
you  over  these  wonderful  and  complica- 
ted phenomena.  By  those  principles  the 
various  facts  of  our  science  are  bound  to- 
gether into  an  organic  whole.  But  we 
have  not  yet  exhausted  the  f  ruitf  ulness  of 
this  principle. 

Consider  the  following  problem. 
Usually  we  allow  the  electricity  of  the 
outer  coating  to  escape  to  the  earth. 
Suppose  we  try  to  utilize  it.  Place,  then, 
your  jar,  A  B,  fig.  40,  upon  vulcanized 
india-rubber,  and  connect  by  a  wire  B  c 
its  outer  coating  with  the  knob  or  inner 
coating  of  a  second  jar  c  D.  "What  will 
occur  when  the  first  jar  is  charged  ? 
Why,  the  second  one  will  be  charged  also 


by  the  electricity  which  has  escaped  from 
the  outer  coating  of  the  first.  And  sup- 
pose you  connect  the  outer  coating  of 
the  second  insulated  jar  with  the  inner 


518 


LESSONS  IN  ELECTRICITY. 


coating  of  a  ifiird,  E  F  ;  what  occurs  ? 
The  third  jar  will  obviously  be  charged 
with  the  electricity  repelled  from  the 
outer  coating1  of  tho  second.  Of  course 
we  need  not  stop  here.  We  may  have  a 
long  series  of  insulated  jars,  the  outer 
coating  of  each  being  connected  with  the 
inner  coating  of  the  next  succeeding  one. 
Connect  the  outer  coating  of  the  last  jar 
i  K  by  a  wire  c  with  the  caith,  and  charge 
the  first  jar.  You  charge  thereby  the 
entire  series  of  jars.  In  this  simple  way 
you  master  practically,  and  grasp  tho 
theory  of  Franklin's  celebrated  "  cascade 
battery." 

You  must  see  that  before  making  this 
important  experiment  you  could  really 
have  predicted  what  would  occur.     This 
power   of  prension    is  one  of  the  most 
striking    characteristics  of  science. 

§  22.  Novel  Ley  den  Jars  of  the  Simplest 
J^orm. 

Possessed  of  its  principles,  we  can  re- 
duce the  Lcydon  jar  to  far  simpler  forms 
than  any  hitherto  dealt  with.  Spread  a 
sheet  of  tin-foil  smoothly  upon  a  table, 
and  Jay  upon  the  foil  a*  pane  of  glass. 
Remember  that  the  glass,  as  usual,  must 
be  dry.  Stick  on  to  the  glass  by  seal- 
ing-wax two  loops  of  narrow  silk  ribbon, 
by  which  the  pane  may  be  lifted  ;  and 
then  lay  smoothly  upon  the  glass  n  sec- 
ond sheet  of  tin-foil,  less  than  the  pane 
in  size,  leaving  a  rim  of  uncotercd  glass 
all  round.  Carry  a  fine  wire  from  the 
upper  sheet  cf  tin-foil  to  your  electro- 
scope. A  little  weight  will  keep  the  end 
of  the  wire  attached  to  the  tin-foil. 

Rub  this  weight  with  your  excited 
glass  tube,  two  or  three  times  if  neces- 
sary, until  you  see  a  slight  divergence  of 
the  Dutch  metal  leaves.  Or  connecting 
the  weight  with  the  conductor  of  your 
machine,  turn  very  carefully  until  the 
slight  divergence  is  observed.  What  is 
the  condition  of  things  here  ?  You  have 
poured,  say  positirc  electiicity  on  to  the 
upper  sheet  of  metal.  It  acts  "inducti  vo  ly 
across  the  glass  upon  the  under  sheet,  the 
positive  fluid  of  which  escapes  to  the 
earth,  leaving  the  negative  behind.  You 
see  before  your  mind's  eye  twx>  layers 
holding  each  other  in  bondage.  Now 
take  hold  of  your  loops  and  lift  the  glass 
plate,  so  as  to  separate  the  upper  tin-foil 
from  the  lower.  What  would  you  ex- 


pect to  occur  ?  Freed  from  the  grasp  of 
the  lower  layer,  the  electricity  of  the  up- 
per one  will  diffuse  itself  over  the  elec- 
troscope so  promptly  and  powerfully, 
that  if  you  are  not  careful  you  will  de- 
stroy the  instrument  by  the  mutual  repul- 
sion of  its  leaves. 

Practise  this  experiment,  which  is  a 
very  old  one  of  mine,  by  lowering  and 
lifting  the  glass  plate,  and  observing  the 
corresponding  rhythmic  action  of  the 
leaves  of  the  electroscope. 

Common  tin-plate  may  be  used  in  this 
experiment  instead  of  tin-foil,  and  a 
sheet  of  vulcanized  india-rubber  instead 
of  the  pano  of  glass.  Or  simpler  still, 
for  the  tin-foil  a  sheet  of  common  un- 
warmed  foolscap  may  be  employed. 
Satisfy  yourself  of  this.  Spread  a  sheet 
of  foolscap  on  a  table  ;  lay  the  plate  of 
glass  upon  it,  an  I  spread  a  leaf  of  fools- 
cap, less  than  the  glass  in  size,  on  the 
plate  of  glass.  Connect  tho  loaf  with 
the  electroscope,  and  charge  it,  exactly 
as  you  charged  the  tin-foil.  On  lifting 
tho  glass  with  its  leaf  of  foolscap,  the 
leaves  of  the  electroscope  instantly  fly 
apart  ;  on  lowering  the  glass  they  again 
fall  together.  Abandon  the  under  sheet 
altogether,  and  make  the  table  the  outer 
coating  ;  if  it  be  not  of  very  dry  wood, 
or  corered  by  an  insulating  varnish,  you 
will  obtain  with  it  the  results  obtained 
with  the  tin-foil,  tin,  and  foolscap. 
Thus  by  the  simplest  means  we  illustrate 
great  principles. 

The  withdrawal  of  the  electricity  from 
the  electroscope,  by  lowering  the  plate  of 
glass,  so  as  to  bring  the  electricity  of  the 
upper  coating  within  the  grasp  of  the 


FIG.  41. 


LESSONS  IN  ELECTRICITY. 


31$ 


lower  one,  is  sometimes  called  **  conden- 
sation. ' '  The  electricity  on  one  plate  or 
sheet  was  figured  ai  squeezed  together, 
^r  condensed,  by  the  attraction  of  the 
other.  A  special  instrument  called  a 
condenser  is  constructed  by  instrument 
makers  to  illustrate  the  action  here  ex- 
plained. 

You  may  readily  make  a  condenser  for 
yourself.  Take  two  circles,  p  p',  fig.  41, 
of  tin  or  of  sheet  zinc,  and  support  the  one, 
p',  by  a  stick  of  sealing-wax  or  glass,  G  : 
the  other,  p,  by  a  metal  stem,  connected 
with  the  earth.  The  insulated  plate,  p', 
is  called  the  collecting  plate  ;  the  unin- 
sulated one,  P,  the  condensing  plate. 
Connect  the  collecting  plate  with  your 
electroscope  by  the  wire  w,  and  bring  tha 
condensing  plate  near  it,  leaving,  how 
ever,  a  thin  space  of  air  between  them. 
Charge  the  collector,  p',  or  the  wire,  w, 
with  your  glass  rod,  until  tho  leaves  of 
the  electroscope  begin  to  diverge. 
Withdraw  the  condensing  plate,  the 
leaves  fly  asunder  ;  bring  tho  condensing 
plate  near,  the  leaves  again  collapse. 

Or  vary  your  constraction,  Rnd  make 
your  condenser  thus.  Employing  the 
table,  or  a  sheet  of  foolscap  if  the  table 
V>e  an  insulator,  as  one  plate  of  the  con- 
denser, sproad  upon  it  the  sheet  of  india- 


rubber,  P,  fig.  42,  and  lay  upon  the 
rubber  the  sheet  of  block-tin,  A  B.  Con- 
nect the  tin  by  the  wire,  w,  with  the 
electroscope,  T.  Impart  electricity  to 
the  little  weight,  A,  till  the  leaves,  L,  W- 
gin  to  diverge  ;  then  lift  the  tin  plate  by 
ks  two  silk  loops  ;  tho  leaves  at  once  fly 
csander. 

Finally,  show  your  complete  knowledge 
of  the  Ley  den  jar,  ami  your  freedom 
from  the  routine  of  the  instrument  makers, 
by  making  a  "jar"  in  the  following 
novel  way.  Stand  upon  a  board  sup- 
ported by  warm  tumblera.  Hold  in  your 
right  hand  a  sheet  of  vulcanized  india- 
rubber,  and  clasp,  with  it  between  you, 
tho  left  hand  of  a  friend  in  connection 
with  the  earth.  Place  your  left  hand  on 
tho  conductor  of  the  machine,  and  let  it 
bo  worked.  You  and  your  friend  sooa 
feel  a  crackling  and  a  tickling  of  the 
hands,  due  to  the  heightening  attraction 
of  the  opposite  electricities  across  the  in- 
dia-rubber. The  "  hand- jar"  is  then 
charged.  To  discharge  it  you  have  onJy 
to  bring  your  other  hands  together  :  ths 
shock  of  the  Leyden  jar  is  then  felt  and 
its  spark  seen  and  heard. 

By  the  discharge  of  the  hand-jar  you 
can  tire  gunpowder.  But  this  will  be  re- 
ferred to  more  particukirly  further  OB. 
(See  §  25.) 

§  23.   Seat  of  Charge  in  the  Ley  den  Jar. 

Franklin  sought  to  determine  how  tho 
charge  was  hidden  in  the  Leyden  jiir. 
He  charged  with  electricity  a  bottle  half 
filled  with  water  and  coated  on  the  out- 
side with  tin-foil  ;  dipping  the  finger  oJ 
one  hand  into  the  water,  and  touching 
the  outside  coating  with  the  other,  he 
received  a  shock.  He  was  thus  led  to 
inquire,  Is  the  electricity  in  the  water  I 
He  poured  the  water  into  a  second  bot- 
tle, examined  it,  and  found  that  it  had 
carried  no  electricity  along  with  it. 

His  conclusion  wa3  '*  that  the  electric 
fire  must  either  have  been  lost  in  tho  d&- 
canting,  or  must  have  remained  in  the 
bottle.  The  latter  he  found  to  be  true  ; 
for,  filling  the  charged  bottle  with  fresii 
water,  he  obtained  the  shock,  and  vrra 
therefore  satisfied  that  the  power  of  giv- 
ing it  resided  in  the  glass  itself."* 

*  Priestley's  "  History  of  Electricity," 
Sa  edition,  p.  149 


320 


(An  account  of  Franklin's  discoveries 
•was  given  by  him  in  a  series  of  letters 
addressed  to  "Peter  Collmson,  Esq.* 
F.R.S.,  from  1747  to  1754). 

So  much  for  history  ;  but  yon  arc  to 
verify  the  history  by  repeating  Franklin's 
experiments.  Place  water  in  a  wide 
glass  vessel  ;  place  ;i  second  glass  vessel 
within  the  first,  and  till  it  to  the  same 
height  with  water.  Connect  the  outer 
water  by  a  wire  with  the  earth,  and  the 
inner  water  by  a  wire  with  the  electric 
machine.  One  or  two  turns  furnish  a  suf- 
ficient charge.  Removing  the  inner  wire, 
and  dipping  one  finger  into  the  outside 
and  the-  other  into  the.  inside  water,  a 
smart  shock  is  felt.  This  was  Franklin's 
first  experiment. 

Pass  on  to  the  second.  Coat  a,  glass 
jar  uiih  tin-foil  (not  too  high)  ;  fill  it  to 
the  same  height  with  water,  and  place  it 
on  india-rubber  cloth.  Charge  it  by 
connecting  the  outside  coating  with  the 
earth,  and  the  water  inside  (by  means  of 
a  stem  cemented  to  the  bottom  of  the  vir 
and  ending  above  in  a  knob)  with  an 
electric  machine.  You  obtain  a  bright 
spark  on  discharging.  This  proves  your 
apparatus  to  be  in  good  order. 

Recharge.  Take  hold  of  the  charged 
jar  with  the  india-rubber,  and  pour  the 
water  into  a  second  similar  jar.  No  sen- 
sible charge  is  imparted  to  the  latter. 
Pour  fresh  unelectrified  water  into  the 
first  jar,  and  discharge  it.  The  retention 
of  the  charge  is  shown  by  a  brilliant  spaik. 
Be  careful  in  these  experiments,  or  yon 
will  fail,  as  I  did  at  first.  The  edge  of 
the  jar  out  of  which  the  water  is  poured 
has  to  be  surrounded  by  a  band  of  bibu- 
lous paper  to  catch  the  final  drop,  which, 
trickling  down,  would  discharge  the  jar. 

Experiments  like  those  of  Franklin  are 
now  made  by  rendering  the  coatings  of 
the  Leydcn  jar  movable.  Such  a  jar  be- 
ing charged,  the  interior  coating  may  be 
lifted  out  and  proved  unelectric.  The 
glass  may  then  be  removed  from  the  outer 
coating  and  the  latter  proved  unelectric. 
Restoring  the  jar  and  coatings,  on  con- 
necting the  two  latter,  the  discharge 
passes  in  a  brilliant  spaik. 

Make  a  jar  with  movable  coatings 
thus  :  Roll  cartridge  paper  round  a  good 
flint-glass  tumbler,  c,  fig.  43,  to  within 
about  an  inch  of  the  top.  Paste  down 
the  lower  edge  of  the  paper,  and  put  a 


LE33O:73  IN  ELECTRICITY. 

paper  bottom  to  it  corresponding  to  the 
bottom  of  the  glass.  Coat  the  paper,  T, 
inside  and  out  with  tin-foil.  Make  a 
similar  coating,  T',  for  the  inside  of  the 
tumbler,  attaching  to  it  an  upright 


PIG.  43. 


w,  ending  in  a  hook.  You  have  then  to 
all  intents  and  purposes  a  Leyden  jar. 

Put  the  pieces  together  and  charge  the 
jar.  By  means  of  a  rod  of  glass,  seal- 
ing-wax, or  gutta-percha,  lift  out  the  in- 
terior coating.  It  will  carry  .a  little  elec- 
tricity away  with  it.  Place  it  upon  a 
tnble  and  discharge  it  wholly.  Then  by 
the  hand  lift  the  glass  out  of  the  outer 
coating.  Neither  of  the  coatings  now 
shows  the  slightest  symptom  of  electric- 
ity. Restore  the  tumbler  to  its  outer 
coating,  and  by  means  of  the  hook  and 
insulating  rod,  restore  the  inner  coating 
to  its  place.  Discharge  the  jar  :  you 
obtain  a  brilliant  spark.  The  electricity 
which  produces  this  spark  must  have 
been  resident  in  and  on  the  glass. 

Here,  as  in  all  other  cases,  you  can 
charge  your  jar  with  a  rubbed  glass  tube, 
though  a  machine  in  good  working  order 
will  do  it  more  rapidly.  With  *'  Cot- 
trell's  rubber,"  described  in  the  next 
section,  you  may  greatly  exalt  the  per- 
formance of  your  glass  tube. 

§  24.  Ignition  by  the  Electric  Spark. 
—  CottrelVs  Rubber.— The  Tube-ma- 
chine. 

Various  attempts  had  been  vainly  made 
by  Nollet  and  others  to  ignite  inflam- 


LESSONS  IN  ELECTRICITY. 


821 


FIG.  44. 

rnable  substances  by  the  electric  spark. 
This  was  first  effected  by  Ludolf,  at  the 
opening  of  the  Academy  of  Sciences  by 
Frederick  the  Great  at  Berlin,  on  the 
23d  of  January,  1744.  With  a  spark 
from  the  sword  of  one  of  the  court  cav- 
aliers present  on  the  occasion,  Ludolf  ig- 
nited sulphuric  ether. 

Dr.  \\atson  also  made  numerous  ex- 
periments on  the  ignition  of  bodies  by 
the  electric  spark.  He  fired  gunpowder 
and  discharged  guns.  Causing,  more- 
over, a  spoon  containing  ether  to  be  held 
by  an  electrified  person,  he  ignited  the 
ether  by  the  finger  of  an  unelectrified  per- 
son. He  also  noticed  that  the  spark  va- 
ried in  color  when  the  substances  between 
which  it  passed  varied. 

These,  and  numerous  other  experi- 
ments may  be  made  with  a  far  simpler 
"  machine"  than  any  hitherto  described. 
It  was  devised  for  your  benefit  by  Mr. 
Cottrell.  In  the  electric  machine,  as  we 
have  learned,  the  prime  conductor  is 
flooded  with  positive  electricity  through 
the  discharge  •  of  the  negative  from  the 
points  against  the  excited  glass.  Your 
gloss  tube  and  rubber  may  be  similarly 
turned  to  account.  A  strip  of  sheet- 
brass  or  copper,  p,  fig.  44,  is  sewn  on  to 
the  edge  of  the  silk  pad,  R,  employed  as 
a  rubber.  Through  apertures  in  the  strip 
about  twenty  pin-points  are  introduced, 
and  soldered  to  the  metal.  When  the 
tube  is  clasped  by  the  rubber,  the  rnetal 
strip  and  points  quite  encircle  the  tube. 

When  a  fine  wire,  ID,  connects  the  strip 
of  metal  with  the  knob  of  a  Leyden  jar, 
by  every  downward  stroke  of  the  rubber 
the  glass  tube  is  powerfully  excited,  and 


hotly  following  the  exciting  rubber  is  t^ 
circle  of  points.  From  these,  against  ths 
rod,  negative  electricity  is  discharged,, 
the  free  positive  electricity  escaping  along; 
the  wire  to  the  jar,  which  is  thus  rapid- 
ly charged. 

The  ignition  of  gas  is  readily  effected! 
by  Cottrell's  rubber.  Connecting  thee 
strip  of  metal,  R,  fig.  45,  with  an  insulat- 
ed metallic  knob,  B,  placed  within  a* 
quarter  or  an  eighth  of  an  inch  of  an 
uninsulated  argand  burner  connected  with 
the  earth,  at  every  downward  stroke  of" 
the  rubber  a  stream  of  sparks  passes  be- 
tween the  knob  and  burner.  If  gas  bo* 
turned  on,  it  is  immediately  ignited  by 
the  stream  of  sparks.  Blowing  out  the- 
fiame  and  repeating  the  experiment,  every 
stroke  of  the  rubber  infallibly  ignites  the- 
£as. 

Sulphuric  ether,  in  a  spoon  which  ha*. 
been  previously  warmed,  is  thus  ignited  ;; 
but  the  ether  soon  cools  by  evaporation  ; 
its  vapor  is  diminished  by  the  cold,  and 
it  is  then  less  easy  to  ignite.  Bisulphide 
of  carbon  may  be  substituted  for  the 
ether,  with  the  certainty  that  every  stroke 
of  the  rubber  will  set  it  ablaze.  The 
spark  thus  obtained  also  fires  a  mixture 
of  oxygen  and  hydrogen.  The  two  gasei 


LESSONS  IF  ELECTRICITY. 


unite  with  explosion  to  form  water,  when 
an  electric  spark  is  pnssed  through  them. 

Mr.  Cottrell  has  also  mounted  his  glass 
-tube  so  as  to  render  friction  in  both  direc- 
jtions  available.  The  tube-machine  is 
represented  in  fig.  46.  A  D  is  the  glass 
tube,  ciasped  by  the  rubber,  n.  p  p1  arc 
two  strips  of  metal  furnished  with  rows 
of  points.  From  p  p'  wires  proceed  to 
the  knob  c,  which  is  insulated  by  the 
horizontal  stem,  o.  This  insulating  stem 
may  be  abolished  with  advantage,  the 
wires  from  p  and  p'  being  rendered  strong 
enough  to  support  the  ball  c.  At  c 
sparks  may  be  taken,  a  Ley  den  jar 
charged,  the  electric  mill  turned,  while 
wires  carried  from  it  may  be  employed 
in  experiments  on  ignition.  I  however 
strongly  recommend  to  your  attention 
the  more  simple  rubber  shown  in  fig.  44. 

"  Seldom,"  says  Hies*,  "  has  an  ex- 
periment done  so  innch  to  'develop  the 
science  to  which  it  belongs  a*  this  of  the 
ignition  of  bodies  by  the  electric  sparks." 
It  aroused  universal  interest  ;  and  was 
repeated  in  all  Royal  houses.  Money 
was  ready  for  the  further  prosecution  of 
electrical  research.  The  experiment 
afterward  spread  among  the  people. 
Jliesw  considers  it  probable  tlu-it  the  gen- 
eral interest  thus  excited  led  to  th«  dis- 
covery of  the  Ley  de  u  jar,  which  was  mado 
soon  after vr&rd. 


Fio.  47. 

Klingenstierna  astonished  King  Fred- 
erick of  Sweden  by  igniting  a  spoon  of 
alcohol  with  a  piece  of  ice.  With  Cot- 
trell's  rubber  and  bisulphide  of  carbon 
this  striking  experiment  is  easily  made, 
and  you  ought  to  render  your  knowl- 
edge complete  by  repeating  it.  At 
every  stroke  of  the  rubber  the  spark 
from  the  end  of  a  pointed  rod  of  ice  in- 
fallibly sets  the  bisulphide)  on  tire. 

Cadogan  Morgan,  in  1785, sought  to  pro- 
duce the  electric  spark  in  the  interior  of 
solid  bodies.  He  inserted  two  wires  into 
wood,  and  caused  the  spark  to  pass  be- 
tween them:  the  wood  \rns  illnminated  with 
blood-red  light,  or  with  yellow  light,  ac- 
cording as  the  depth  at  which  the  spark 
was  produced  wa~s  greater  or  less.  Tlia 
spark  of  the  Leyden  jar  produced  within  an 
ivory  ball,  an  orange, an  apple, or  under  tin; 
thumb,  illuminates  these  bodies  through- 
out. A  lemon  is  especially  suited  to  this 


ffra,  48. 


LESSONS  IN  ELECTRICITY. 


experiment,  flashing  forth  at  every  spark 
as  a  spheriod  of  brilliant  golden  Jight. 
The  manner  in  which  the  lemon  is  mount- 
ed on  the  brass  stem  B  is  shown  in  fig. 
47.  The  spark  occurs  at  5,  in  the  interval 
between  the  stems  A  and  B.  A  row  of 
eggs  in  a  glass  cylinder  is  also  brilliantly 
illuminated  at  the  passage  of  every  spark 
from  a  Ley  den  jar. 

§  25.   Duration  of  the  Electric  Spark. 

The  duration  of  the  electric  spark  is 
very  brief  ;  in  a  special  case  Sir  Charles 
Whcatstone  found  it  to  be  jrj^^tli  of  a 
second.  This,  however,  was  the  maxi- 
mum duration.  In  other  cases  it  was 
less  than  the  millionth  of  a  second. 

When  a  body  is  illuminated  for  a«  in- 
stant, the  image  of  the  body  remains 
upon  the  retina  of  the  eye  for  about  one- 
fifth  of  a  second.  If,  then,  ji  body  in 
swift  motion  be  illuminated  by  an  instan- 
taneous flash,  it  will  be  seen  to  stand 
motionless  for  one-fifth  of  a  second  at 
the  point  where  the  flash  falls  upon  it.  A 
riiie  bullet  passing  through  the  air,  and 
illuminated  by  an  electric  flash,  would 
be  seen  thus  motionless  ;  a,  circle  like  D 
D',  fig.  48,  divided  into  black  and  white 
sectors,  and  rotating  so  quickly  as  to 
cause  the  sectors  to  blend  to  a  uniform 
gray,  appears,  when  illuminated  by  the 
spark  of  a  Leyden  jar,  perfectly  motion- 
less, with  all  its  sectors  revealed.  A  fall- 
ing jet  of  water,  which  appears  contin- 


uous, is  resolved  by  'the  electric  flash 
into  its  constituent  drops.  Lightning, 
as  shown  by  Professor  Dove,  is  similarly 
rapid  in  its  discharge. 

For  a  long  time  it  was  found  almost 
impossible  to  ignite  gunpowder  by  the 
electric  spark.  Its  duration  is  so  brief 
that  the  powder,  when  the  discharge  oc- 
curred in  its  midst,  was  simply  scattered 
violently  about.  In  1787  Wolff  intro- 
duced into  the  circuit  through  which  the 
discharge  passed  a  glass  tube  wetted  on 
the  inside.  He  thereby  rendered  the 
ignition  certain.  This  was  owing  to  the 
retardation  of.  the  spark  by  the  imperfect 
conductor.  Gun-cotton,  phosphorus,  and 
amadou,  which  are  torn  asunder  by  the 
unrelarded  sparks  are  ignited  when  the 
discharge  is  retarded  by  a  tube  of  water. 
A  wetted  string  is  the  usual  means  re- 
sorted to  for  retardation  when  gunpow- 
der is  to  be  discharged. 

The  instrument  usually  employed  for 
the  ignition  of  powder  is  the  universal 
discharger.  We  make  our  own  dis- 
charger thus  :  i  and  ^-(fig.  49)  are  in- 
sulating rods  of  glass  or  sealing-wax, 
supporting  two  metal  arms,  the  ends  of 
which  can  be  brought  down  upon  the 
little  central  table  s.  One  of  the  metal 
arms  of  the  discharger  being  connected 
by  a  wire  e  with  the  eartL,  the  separated 
ends  of  the  two  arms  are  surrounded 
^Yith  powder  B.  Sending  through  it  the 
unretarded  charge,  the  powder  ia  scatter- 


FIQ.  49. 


824 


LESSONS  IN  ELECTRICITY. 


FIG.  50. 

ed  mechanically.  Introducing  the  wet 
string  w  into  the  circuit,  ignition  infalli- 
bly occurs  when  the  spark  passes. 

This  is  the  place  to  fulfil  our  promise 
to  ignite  gunpowder  by  the  "  hand- jar. " 
Fig.  50  explains  the  arrangement.  H  11' 
are  the  hands  of  the  insulated  person. 
F  the  hand  of  the  uninsulated  friend,  i 
(he  india-rubber  between  both  hands. 
The  lead  ball  B  is  suspended  by  a  wet 
string  s.  On  the  little  stand  P,  connect- 
ed with  the  earth,  is  placed  the  powder. 
The  charging  of  the  hand- jar  is  described 
in  §  22.  When  charged,  it  is  only  nec- 
essary to  bring  the  ball  n  down  upon 
the  powder  to  cause  it  to  explode. 

§  26.   Electric  Light  in  Vacuo. 

The  electric  light  in  vacuo  was  first 
observed  by  Picard  in  1675.  While 
carrying  a  barometer  from  the  Observa- 
tory to  the  Porte  St.  Michel  in  Paris,  he 
saw  light  in  the  upper  portion  of  the 
lube.  Sebastien  and  Cassini  observed  it 
Afu?r wards  in  other  barometers.  John 
iicrnouilli  devised  a  "  mercurial  phos- 
phorus," by  shaking  mercury  in  a  tube 
vhlch  had  been  exhausted  by  an  air- 
pump.  This  was  handed  to  the  King  of 
Prussia — Frederick  I. — who  awarded  for 
it  a  medal  of  forty  ducats  value.  The 
great  mathematician  wrote  a  poem  in 
noiior  of  the  occasion. 


Fia.  51. 

Bernouilli  failed  to  explain  the  effect. 
The  explanation  was  reserved  for  Ilauks- 
bee,  who  in  1705  took  up  the  subject 
and  experimented  upon  it  before  the 
Royal  Society.  On  tke  plate  of  an  air- 
pump  he  placed  two  bell-jars,  one  over 
the  other.  Tin  outer  and  larger  jar  was 
open  at  Hie  top.  Into  the  opening 
llauksbee  iixod,  air-tight,  a  funnel, 
which  he  stopped  with  a  plug  of  wood 
and  filled  with  mercury,  lie  exhausted 
the  space  between  the  two  jars,  withdrew 
the  wooden  plug  and  allowed  the  mer- 
cury to  stream  against  the  outer  surfacei 
of  the  inner  jar.  lie  thus  obtained  aJ 
shower  of  lire.  This  is  a  truly  beautiful1 
experiment  when  witnessed  by  an  ob- 
server close  at  hand. 

A  copy  of  Hauksbee's  own  figure  il- 
lustrating this  experiment  is  annexed, 
fig.  51.  M  is  the  funnel  containing  the 
mercury,  i»the  plug  of  wood,  8  the.  outer 
and  s'  the  inner  bell-jar.  Instead  of  the 
plug  P,  an  india-rubber  tube,  held  by  a 
clip,  may  be  employed  with  advantage  to 
connect  the  funnel  with  the  exhausted 
jar.  By  gradually  relaxing  the  clip  the 
mercury  may  be  made  to  fall  at  a  rate 
corresponding  to  the  maximum  luminous 
effect.  The  streams  of  light  produced 
are  very  beautiful,  but  they  arc  more 
continuous  than  they  arc  shown  to  be  by 
Uauksbuo, 

In  1706   llauksbee  referred  the  phe- 


LESSONS  IN  ELECTRICITY. 


nomenon  to  •  its  true  cause,  namely,  the 
friction  between  mercury  and  glass  in  the 
highly  rarefied  air.  John  Bernoulli!  ridi- 
culed Hauksbee's  explanation.  But 
truth  outlives  ridicule,  and  it  is  now  uni- 
versally admitted  that  Hauksbee  was 
right. 

Hauksbee  also  made  the  following  ex- 
periment, which,  as  shown  by  Riess,  is 
explained  by  reference  to  the  principle  of 
induction.  A  hollow  glass  globe  was 
mounted  so  as  to  be  capable  of  quick 
rotation.  It  was  exhausted,  and  while 
it  rotated  the  hand  was  placed  against  it 
in  the  dark.  It  was  positively  electri- 
fied by  the  hand.  This  positive  electricity 
acted  inductively  on  the  gla?s  itself,  at- 
tracting its  negative,  but  discharging  its 
positive  as  a  luminous  g'ow  through  the 
rarefied  air  within.  Haukshee  was  able 
to  read  by  the  light  thus  produced. 

By  such  experiments  it  was  shown  that 
rarefied  air  favored  the  passage  of  elec- 
tricity. Dry  air  is  in  fact  an  insulator, 
which  must  be  broken  through  to  pro- 
duce the  electric  spark.  Through  an  ex- 
hausted glass  tube  six  feet  long  a  dis- 
charge freely  passes  which  would  be  in- 
competent to  leap  over  the  fiftieth  part 
of  this  interval  in  air.  But  whereas  the 
spark  in  air  is  dense  and  brilliant,  the 
discharge  in  vacuo  fills  the  exhausted 
tube  with  a  diffuse  light. 

(It  is  here  worthy  of  remark  that  at 
a  very  early  period  Grummert,  a  Pole, 
proposed  the  employment 'of  this  diffuse 
electric  light  to  illuminate  coal  mines — a 
notion  which  has  been  revived  in  our 
day.  The  light  in  this  form  is  not  com- 
petent to  ignite  the  explosive  gases  which 
produce  such  terrible  disasters  in  mines.) 

Priestley,  in  his  "  History  of  Electric- 
ity," thus  describes  the  light  in  vacuo. 
"  Take  a  tall  receiver,  very  dry,  and  in  the 
top  of  it  insert  with  cement  a  wire  not 
very  acutely  pointed,  then  exhaust  the 
receiver  and  present  the  knob  of  the  wire 
to  the  conductor,  and  every  spark  will 
pass  through  the  vacuum  in  a  broad 
stream  of  light,  visible  through  the  whole 
length  of  the  receiver,  be  it  ever  so  tall. 
This  stream  often  divides  itself  into  a 
variety  of  beautiful  rivulets,  which  are 
continually  changing  their  course,  uniting 
and  dividing  again  in  the  most  pi-easing 
manner.  If  a  jar  be  discharged  through 


this  vacuum,  it  gives  the  appearance  of  a 
very  dense  body  of  fire,  darting  directly 
through  the  centre  of  the  vacuum  with- 
out ever  touching  the  sides." 

Cavendish  employed  a  double  barome- 
ter-tube, bent  into  a  form  of  a  horseshoe, 
with  its  curved  portion  empty,  to  show 
the  passage  of  electricity  through  a 
vacuum.  It  is  really  not  the  vacuum 
which  conducts  the  electricity,  but  the 
highly  attenuated  air  and  vapor  which 
fill  the  space  above  the  barometric 
columns.  When  the  mercury  employed 
is  carefully  purged  of  air  and  moisture 
by  previous  boiling,  the  space  above 
the  mercury,  as  proved  by  Walsh, 
De  Luc,  Morgan,  and  Davy,  is  wholly 
incapable  of  conducting  electricity. 
Similar  experiments  have  been  made  in 
the  laboratory  of  Mr.  Gassiot,  to  whom 
we  are  indebted  for  so  many  beautiful 
electrical  experiments.  Professor  Dewar 
has  also  brought  his  experimental  skill  to 
bear  with  success  upon  this  subject. 

Electricity,  therefore,  dees  not  pass 
through  a  true  vacuum  ;  it  requires  pon- 
derable matter  to  carry  it.  If  a  gold- 
leaf  electroscope  be  kept  at  a  distract 
from  al]  conductors,  it  may  be  kept 
charged  for  an  almost  indefinite  period 
in  a  good  air-pump  vacuum. 

The  matter  rendered  thus  luminous  by 
the  electrical  discharge  is  attracted  and 
repelled  like  other  electrified  matter.  "  A 
finger,"  says  Priestley,  "  put  on  the  out- 
side of  the  glass  will  draw  it  [the  lumi- 
nous stream]  wherever  a  person  pleases. 
If  the  vessel  be  grasped  with  both  hands, 
every  spark  is  felt  like  the  pulsation  of  a 
great  artery,  and  all  the  fire  makes  to- 
wards the  hands.  This  pulsation  is  felt 
at  some  dir»tance  from  the  receiver  ;  and 
in  the  dark  a  light  is  seen  betwixt  the 
hands  and  glass." 

"  All  this,"  continues  the  historian  of 
electricity,  "  while  the  pointed  wire  is 
supposed  to  be  electrified  positively  ;  if 
it  be  electrified  negatively  the  appearance 
is  remarkably  different.  Instead  of 
streams  of  fire,  nothing  is  seen  but  one 
uniform  luminous  appearance,  like  a 
white  cloud,  or  the  milky-way  on  a  clear 
starlight  night.  It  seldom  readies  the 
whole  length  of  the  vessel,  but  is  gen- 
erally only  like  a  lucid  ball  at  the  end  of 
the  wire." 

Of  the  two  appearances  here  described 


320 


LESSOVS   IN   ELECTRICITY. 


FKI.  &1 

tho  fonriGr  is  now  known  n«  tho  ehctrie 
brush,  and  the  latter  rs  the  electric  glow. 
Both  c*n  be  produced  in  unconfined  air. 
Tho  glow  is  sometimes  seen  on  the  masts, 
of  clips,  and  it  is  mentioned  by  tho  an- 
ci-euta  as  appearing  on  the  points  of 
kacea.  It  is  called  St.  Ermo's  or  St. 
Elmo's  fire,  ?:ftcr  the  .*ailoiV  saint,  Eras- 
inu*,  \*ho  nuHered  martyrdom  at  Gaeta, 
at  the  beginning  of  the  fourth  century. 

The  purple  color  of  the  diffused  light 
in  attenuated  air  was  noticed  by  Hauks- 
boc.  The  color  depends  upon  the  resi- 
due of  attenuated  gas,  or  vapor,  through 
which  the  discharge  passes.  If  it  be  an 
oxygen-residue  the  light  is  whitish,  if  it 
be  a  hydrogen-residue  the  light  is  red,  if 
a  nitrogen-residue  the  light  is  purple, 
fj;act!y  resembling  that  displayed  at  times 
by  vlro  aurora  borealis — a  color  doubtless 
due  to  tho  discharge  of  electricity 
through  the  attenuated  nitrogen  of  the  air. 

Electric  light  in  vacuo  is  readily  pro- 
duced by  the  friction  of  an  amalgamated 
FH  I -^cr  against  the  outside  of  an  exhausted 
tube.  The  light  is  also  produced  by  the 
friction  of  mercury  within  a  barometric 
vacuum.  The  discharges  through  tube* 


many  feet  in  length  and  exhausted  by 
an  air-pump  are  very  fine.  The  double 
barometer  tube  of  Cavendish  also  yields 
a  truly  splendid  bow  of  light,  when  a 
strong  electric  discharge  is  sent  through 
it.  For  this  experiment  fig.  52  shows 
the  best  arrangement.  p  is  the  prime 
conductor  of  an  electrical  machine,  i  an 
insulated  metal  ball,  connected  by  a  wire 
with  the  mercury  trough  A.  The 
trough  u  is  connected  by  a  wire  with  the 
earth,  c  and  c'  mark  the  height  of  the 
mercurial  columns.  When  the  machine 
is  worked  sparks  pass  from  p  to  i,  a 
vivid  bow  of  light  at  each  passage  stretch- 
ing from  o  to  c'.  By  causing  i  to  ap- 
proach P,  the  discharges  become  more 
frequent,  t>nt  more  feeble  ;  by  augment- 
ing the  distance  P  i,  the  sparks  become 
rarer,  but  more  strong.  When  very 
»tro.ng,  a  bow  of  dazzling  brilliancy  ac- 
companies every  spark.* 

Small  tubes  tor  these  experiments  are 
hd*t  obtained  from  philosophical  instru- 
ment makers 

§  27.   Lichlenbtry's  Figures. 

Licht.cnberg  deriasd  a  me-xis  of  rc- 
vaaling  tho  condition  of  an  electrified 
surface  by  dusting  it  with  powder.  It'sd 
lead,  in  passing  through  mr-slin,  is  p</«i- 
tivcly  electrified  ;  flower  of  sulphur  M 
negatively  electrified.  Whisking  a  fox's 
brush  over  a  cake  of  resin,  and  drawing 
over  the  surface  the  knob  of  a  Lcyden  j*r, 
positively  charged,  tho  re^.n  is  rendered 
in  part  negafivc  and  in  part  positive. 
Dusting  the  mixed  powder  over  the  sur- 
face, the  Rulphur  arrange*  itself  over  the 
positive  places,  and  the  red  lead  ovc? 
the  negative  places,  a  verv  beautiful  pat- 
tern being  the  result. 

This  experiment  of  LicMenberg's  con- 
stituted the  germ  of  Chladni's  important 
acoustical  researches.  k  Chladni's  fig- 
ures "  were  the  direct  offspring  of 
**  Lichtcnbcrg's  figures." 

§  28.  Surface  Compared  with  Mass. 
Distribution  cf  Electricity  in  Hollow 
Conductors. 

Monnier  proved  that  the  charge  of  a 

*  It  is  well  ta  have  tho  interval  p  i  fit 
s"»me  distance  from  the  how,  srvthr.t  Ihu  liirht 
of  the  spark  shall  not  impair  thu  efl(x:t  of  ih« 
discharge  upon  the  eye. 


LESSONS  IN  ELECTRICITY. 


327 


conductor  depended  upon  its  surface, 
and  not  upon  its  solid  content*.  An 
anvil  weighing  200  pounds  gave  a  smaller 
sp::rk  tl«an  a  speaking  trumpet  weighing 
10  pounds.  A  solid  ball  of  lead  gave  a 
spark  only  of  the  same  force  as  that  ob- 
tained from  a  piece  of  thin  lead  of  tho 
same  superficies,  bent  into  the  form  of  a 
hoop.  Finally  Monnier  obtained  a  strong 
spark  from  a  long  strip  of  sheet  lead, 
but  a  very  small  one  when  it  was  rolled 
into  a,  lump. 

Le  Roi  and  D'Arcy  showed  that  a  hol- 
low sphere  accepted  the  same  charge 
when  empty  as  when  filled  with  mercury, 
which  augmented  its  weight  sixty-fold. 
And  this  proves  the  influence  of  surface 
ad  distinguished  from  mass. 

The  distribution  of  electricity  is  well 
illustrated  by  the  deportment  of  hollow 
bodies.  Impart  by  your  carrier  (fig.  15) 
successive  measures  of  electricity  to  the 
interior  of  an  insulated  ice-pail,  or  a 
pewter  pot.  On  testing  the  interior  of 
the  vessel  with  the  carrier  and  an  elec- 
troscope no  •  electricity  is  found  there  ; 
but  it  is  found  on  the  external  surface. 
A  hat  suspended  by  silk  strings  answers  as 
well  as  the  ice-pail. 


Thi*  ^Tperiment  with  the  hat  is  a  very 
inatractire  one.  Tho  hat  may  be  charged 
cither  with  Cottrell's rubber  or  with  your 
rubbed  glass  tube. 

Notice,  Trhen  testing,  that  you  take 
your  strongest  charges  from  the  cd^ss 
and  not  from  tho  round  or  flat  .smface  cf 
the  hat.  The  strongest  charge  of  all  U 
communicated  to  the  earlier  by  the  IG?£ 
of  the  hat. 

The  successive  charges  may  be  cow- 
muni  cated  to  the  hat  by  a  metal  ball  :»u-:> 
pcndcd  by  silk.  The  charged  b«ll,  o* 
touching  the  interior  surface,  beeorr>4» 
completely  unclectric. 

Franklin  placed  a  long  chain  in  -A  sti- 
ver tea-pot  which  he  electrified.  Con- 
necting his  teapot  with  a  pith-ball  ch •-.- 
troscope  he  produced  a  diverge*.:*. 
Then  lifting  the  chain  by  a  silk  string**. 
found  that  over  the  portion  outside  v~«$ 
teapot  the  electricity  diffused  itself,  T!M£ 
withdrawal  of  the  electricity  from  !*t* 
electroscope  being  announced  by  the  par- 
tial collapse  of  the  divergent  pith-balls. 

The  mode  of  repeating  lhi»  experiyiiicjj 
is  shown  in  fig.  53,  where  T  is  the  tea- 
pot, supported  on  a  ^ood  glass  tumbler 
o,  and  connected  by  the  wire  w  with  the 


328 


LESSONS  IN  ELECTRICITY. 


electroscope  E.     The  effect  is  small,  but 
distinct. 

The  greatest  experiment  with  hollow 
conductors  was  made  by  Faraday,  who 
placed  himself  in  a  cubical  chamber  built 
of  laths  and  covered  with  paper  and  wire 
gauze.  It  was  suspended  by  silk  ropes. 
Within  this  chamber  he  could  not  detect 
tho  slightest  sign  of  electricity,  however 
delicate  his  electroscope,  and  however 
strongly  the  sides  of  the  chamber  might 
t>e  electrified. 

§  29.    Physiological  Effects  of  the  Elec- 
tric Discharge. 

The  physiological  effect  of  the  electric 
shock  lias  been  studied  in  various  ways. 
Graham  caused  a  number  of  persons  to 
lay  hold  of  the  same  metal  plate,  which 
was  connected  with  the  outer  coating  of 
a  charged  Leyden  jar,  and  also  to  lay 
hold  of  a  rod  by  which  the  jar  was  dis- 
charged. The  shock  divided  itself 
equally  among  them. 

The  Abbe  Nollet  formed  a  lino  of  one 
hundred  and  eighty  guardsmen,  and  sent 
the  discharge  through  them  all.  He  also 
killed  sparrows  and  fishes  by  the  shock. 
The  analogy  of  these  effects  with  those 
produced  by  thunder  and  lightning  could 
not  escape  attention,  nor  fail  to  stimulate 
inquiry. 

Indeed,  as  experimental  knowledge  in- 
creased, men's  thoughts  became  more  def- 
inite and  exact  as  regards  the  relation  of 
electrical  effects  to  thunder  and  lightning. 
The  Abbe  Nollet  thus  quaintly  expresses 
himself  :  "  If  any  one  should  take  upon 
him  to  prove,  from  a  well-connected 
comparison  of  phenomena,  that  thunder 
is,  in  the  hands  of  Nature,  what  electric- 
ity is  in  ours,  and  that  tho  wonders 
which  we  now  exhibit  at  our  pleasure  are 
little  imitations  of  those  great  effects 
which  frighten  us  ;  I  avow  that  this  idea, 
if  it  was  well  supported,  would  give  me 
a  great  deal  of  pleasure.'  lie  then 
points  out  the  analogies  between  both, 
and  continues  thus  :  "  All  those  points 
of  analogy,  which  I  have  been  some  time 
meditating,  begin  to  make  me  believe 
that  one  might,  by  taking  electricity  as 
the  model,  form  to  one's  self  in  relation  to 
thunder  and  lightning,  more  perfect  and 
more  probable  ideas  than  what  have  been 
-offered  hitherto."* 

*  Priestley's  "  Ilistorvof  Electricity,"  pp. 
151-52. 


These  views  were  prcralent  at  tho  time 
now  referred  to,  and  o.ut  of  them  grew 
the  experimental  proof  by  tne  great 
physical  philosopher,  Franklin,  of  the 
substantial  identity  of  the  lightning  Gash 
and  the  electric  spark. 

Franklin  was  twice  struck  senseless  by 
the  electric  shock.  lie  afterwards  sent 
the  discharge  of  two  large  jars  through 
six  robust  men  ;  they  fell  to  the  ground 
and  got  up  again  without  knowing  what 
had  happened  ;  they  neither  heard  nor 
felt  the  discharge.  Priestley,  who  made 
many  valuable  contributions  to  elec- 
tricity, received  the  charge  of  two  jars, 
but  did  not  find  it  painful. 

This  experience  agrees  with  mine. 
Some  timo  ago  I  stood  in  this  room  with 
a  charged  battery  of  fifteen  large  Leyden 
jars  beside  me.  Through  some  awkward- 
ness on  my  part  I  touched  the  wire  lead- 
ing from  the  battery,  and  the  discharge 
went  through  me.  Fur  a  sensible  inter- 
val life  was  absolutely  blotted  out,  but 
there  was  no  trace  of  pain.  After  a  life- 
tie  time  consciousness  returned  ;  I  saw 
confusedly  both  the  audience  and  the  ap- 
paratus, arid  concluded  from  this,  and 
from  my  own  condition,  that  I  had  re- 
ceived the  discharge.  To  prevent  the 
audience  from  being  alarmed,  I  made 
the  remark  that  it  had  often  been  my  de- 
sire to  receive  such  a  shock  accident- 
ally, and  that  my  wish  had  at  length 
been  fulfilled.  But  though  th§  intellect- 
ual consciousness  of  my  position  return- 
ed with  exceeding  rapidity,  it  was  not  so 
with  the  optical  consciousness.  For, 
while  making  the  foregoing  remark,  my 
body  presented  to  my  eyes  the  appear- 
ance of  a  number  of  separate  pieces. 
The  arms,  for  example,  were  detached 
from  the  trunk  and  suspended  in  the  air. 
In  fact,  memory  and  the  power  of  rea- 
soning appeared  to  be  complete,  long  be- 
fore the  restoration  of  the  optic  nerve  to 
healthy  action. 

This  may  be  regarded  as  an  experi- 
mental proof  that  people  killed  by  light- 
ning suffer  no  pain. 

§   80.    Atmospheric  Electricity. 

The  air  at  all  times  can  bo  proved  to 
be  a  reservoir  of  electricity,  which  un- 
dergoes periodic  variation.  We  have 
seen  that  ingenious  men  began  soon  to 
suspect  u  common  on^is.  for  the  crack- 


LESSONS  IN  ELECTRICITY. 


820 


Fio.  54. 


ling  and  light  of  the  electric  spark,  and 
thunder  and  lightning.  The  greatest  in- 
vestigator in  this  field  is  the  celebrated 
Dr.  Franklin.  He  made  an  exhaustive 
comparison  of  the  effects  of  electricity 
and  those  of  lightning.  The  lightning 
flash  he  saw  was  of  the  same  shape  as  the 
elongated  electric  spark  ;  like  electricity, 
lightning  strikes  pointed  objects  in  pref- 
erence to  others  ;  lightning  pursues  the 
path  of  least  resistance  ;  it  burns,  dis- 
solves metals,  rends  bodies  asunder,  and 
strikes  men  blind.  Franklin  imitated  all 
these  effects,  striking  a  pigeon  blind,  and 
killing  a  hen  and  turkey  by  the  electrical 
discharge.  I  place  before  you  in  fig. 
£-?-,  with  a  view  to  its  comparison  with  a 
;«*charge  of  forked  lightning,  the  long 
spark  obtained  from  an  effective  ebonite 
machine,  furnished  with  a  conductor  of  a 
special  construction,  which  favors  length 
of  spark. 

Having  completely  satisfied  his  mind 
by  this  comparison  of  the  identity  of 
both  agents,  Franklin  proposed  to  draw 
electricity  from  the  clouds  by  a  pointed 
rod  erected  oa  a  high  tower.  But  be- 
fore the  tower  could  be  built  he  succeed- 
ed in  his  object  by  means  of  a  kite  with 
a  pointed  wire  attached  to  it.  The 
electricity  descended  by  the  hempen 
string  which  held  the  kite,  to  a  key  at 
the  end  of  it,  the  key  being  separated 
from  the  observer  by  a  silken  string  held 
in  the  hand.  Franklin  thus  obtained 
sparks,  and  charged  a  Leyden  phial  with 
atmospheric  electricity. 

But,  spurred  by  Franklin's  researches, 
an  observer  in  France  had  previously 
proved  the  electrical  character  of  light- 
ning. A  translation  of  Franklin's  writ- 
ings on  the  subject  fell  into  the  hands  of 
the  naturalist  Buffon,  who  requested  his 
friend  D'Alibard  to  revise  the  transla- 
tion. D'Alibard  was  thus  induced  to 
erect  an  iron  rod  40  feet  long,  supported 
by  silk  strings,  and  ending  in  a"  sen  try- 


box.  It  was  watched  by  an  old  dragoon 
named  Coiffier,  who  on  the  10th  of  May, 
1752,  heard  a  clap  of  thunder,  and  im- 
mediately afterwards  drew  sparks  from 
the  end  ot  the  iron  rod. 

The  danger  of  experiments  with  metal 
rods  was  soon  illustrated.  Professor 
Richmann  of  St.  Petersburg  had  a  rod 
raised  three  or  four  feet  above  the  tiles 
of  his  house.  It  was  connected  by  a 
chain  with  another  rod  in  his  room  ;  the 
latter  rod  resting  in  a  glass  vessel,  and 
being  therefore  insulated  from  the  earth. 
On  the  Gth  of  August,  1753,  a  thunder 
cloud  discharged  itself  against  the  exter- 
nal rod  ;  the  electricity  passed  down- 
wards along  the  chain  ;  on  reaching  the 
tod  below,  it  was  stopped  by  the  glass 
vessel,  darted  to  Richmann's  head,  which 
was  about  a  foot  distant,  and  killed  him 
on  the  spot.  Had  a  perfect  communica- 
tion eiisted  between  the  lower  rod  and 
the  earth,  the  lightning  in  this  case  would 
hav«  expended  itself  harmlessly. 

In  1749  Franklin  proposed  lightning 
conductors.  He  repeated  his  recom- 
mendation in  1753.  He  was  opposed  on 
two  grounds.  The  Abbe  Nollct,  and 
those  who  thought  with  him,  considered 
it  as  impious  to  ward  off  heaven's  light- 
nings, as  for  a  child  to  ward  off  the 
chastening  of  its  father.  Others  thought 
that  the  conductors  would  "  invite"  the 
lightning  to  break  upon  them.  A  long 
discussion  was  also  carried  on  as  to 
whether  the  conductors  should  be  blunt 
or  pointed.  Wilson  advocated  blunc 
conductors  against  Franklin,  Cavendish, 
and  Watson.  lie  so  influenced  George 
III.,  hinting  that  the  points  were  a  re- 
publican device  to  injure  his  Majesty, 
that  the  pointed  conductors  on  Bucking- 
ham House  were  changed  for  others  end- 
ing in  balls.  Experience  of  the  most 
varied  kind  has  justified  the  employment 
of  pointed  conductors.  In  1769  St. 


880 


LESSONS  IN  ELECTRICITY. 


Fio.  55. 


Paul's  Cathedral  was  first  protected. 

The  most  decisive  evidence  in  favor  of 
conductors  was  obtained  from  ships  ;  and 
such  evidence  was  needed,  to  overcome 
the  obstinate  prejudice  of  seamen.  Case 
after  case  occurred  in  which  ships  un- 
protected by  conductors  were  singled 
out  from  protected  ships,  and  shattered 
or  destroyed  by  lightning.  '  The  con- 
ductors were  at  first  made  movable,  be- 
ing ioisted  on  the  approach  of  a  thun- 
derstorm ;  hut  these  were  finally  aban- 
doned for  the  fixed  lightning  conductors 
devised  by  the  late  Sir  Snow  Harris. 
The  saving  of  property  and  life  by  this 
obvious  outgrowth  of  electrical  research  is 
incalculable. 

§   31.    TJte  Returntny  Stroke. 

In  the  year  1779  Charles,  Viscount  Ma- 
hon,  afterward  Earl  Stanhope,  pub- 
lished his  "  Principles  of  Electricity." 
On  the  title-page  of  the  book  stands  the 
following  remark  : — "  This  treatise  com- 
prehends an  explanation  of  an  electrical 
returning  stroke,  by  which  fatal  effects 
may  be  produced  even  at  a  vast  distance 
from  the  place  where  the  lightning 
falls." 

Lord  Mahon's  experiments,  which  are 
models  Df  scientific  clearness  and  pre- 
cision, will  be  readily  understood  by  ref- 


erence to  the  principles  of  electric  induc- 
tion, with  which  you  are  now  so  familiar. 
It  need  only  be  noted  here  that  whenever 
he  speaks  of  a  body  being  plunged  in  an 
"  electrical  atmosphere,"  he  means  that 
the  body  is  exposed  to  the  inductive  ac- 
tion of  a  second  electrified  body,  which 
latter  he  supposed  to  be  surrounded  by 
such  an  atmosphere. 

A  few  extracts  from  his  work  will  gir* 
a  clear  notion  of  the  nature  of  his  dis- 
covery : 

"  I  placed  an  insulated  metallic  cylin- 
der, A  B,  fig.  55,  within  the  electrical  at- 
mosphere of  the  prime  conductor  [p  c] 
when  charged,  but  beyond  the  striking 
distance.  The  distance  between  the  near 
end  A  of  the  insulated  metallic  bodj 
and  the  side  of  the  prime  conductor 
was  20  inches.  The  body  A  u  wsa 
of  brass,  of  a  cylindrical  form,  .18  inch- 
es long,  by  two  inches  in  diameter. 
I  then  placed  another  insulated  brass 
body  E  F,  40  inches  long  by  about  3j 
inches  in  diameter,  with  its  end  E  at  ths 
distance  of  about  one-tenth  of  an  inch 
from  the  en-l  B  of  the  other  metallic 
body  A  B.  I  electrified  the  prime  con- 
ductor. All  the  time  that  it  was  receiv- 
ing its  plus  charge  of  electricity  there 
passed  a  great  number  of  weak  (red  or 


LESSONS  IN  ELECTRICITY. 


331 


"mrple)  sparks  from  the  end  u  of  tlio  near 
jodv  A  B  into  the  end  E  of  the  remote 
body  EI-." 

Make  clear  to  your  mind  tiio  origin  of 
thi.s  stream  of  weak  red  or  purple  sparks. 
It  i.i  obviously  dua.  to  the  inductive 
action  of  the  prime  conductor  P  c  upon 
tlio  body  A  c.  The  positive  electricity 
of  A  c  being  repelled  by  the  prims  con- 
ductor, passed  as  a  stream,  of  sparks  to 

E  F. 

"  When  the  prime  conductor,  Laving 
received  its  full  charge,  came  suddenly 
to  discharge,  with  an  explosion,  its  super- 
abundant electricity  on  a  large  brass  ball 
L,  which  was  made  to  communicate  with 
lihe  earth,  it  always  happened  that  the 
electrical  fluid,  which  had  been  gradually 
expelled  from  the  body  A  B  and  driven  into 
the  body  E  r,  did  suddenly  return  from 
the  body  E  F  iuto  the  body  A  n,  in  a 
strong  and  bright  spark,  at  the  very  in- 
stant that  the  explosion  took  place  upon 
the  ball  L. 

"  This  I  call  the  electrical  returning 
stroke." 

For  the  two  conductors  Lord  Mali  on 
then  substituted  his  own  body  and  that 
of  another  person,  both  of  them  standing 
upon  insulating  stools.  He  continues 
thus  : 

**  I  placed  myself  upon  an  insulating 
stool  E  (fig.  56),  so  as  to  have  my  right 
arm  A  at  the  distance  of  about  20  inches 
from  a  large  prime  conductor  ;  another 
person,  standing  upon  another  insulating 


es. 


stool  K,  brought  his  right  hand  F  within 
ono-  quarter  of  an  inch  of  my  left  hand  n. 

41  When  the  prime  conductor  began  to 
receive  its  plus  charge  of  electricity,  we 
felt  the  electrical  fluid  running  out  of  my 
hand  B  into  his  hand  F. 

"  V>Then  we  separated  our  hand?  B  and 
F  a  little,  the  electricity  passed  between 
us  in  small  sparks,  which  sparks  increased 
in  sharpness  the  farther  we  removed  our 
hands  c  and  F  asunder,  until  we  had 
brought  them  quite  out  of  a  striking  dis- 
tance. The  interval*  of  time  between 
these  departing  sparks  increased  also  the 
more  the  distance  between  our  hands  B 
and  Y  was  increased,  as  must  necessarily 
be  the  case. 

"As  soon  as  the  prime  conductor  came 
suddenly  to  discharge  iU  electricity  upon 
the  ball  L,  the  superabundant  electricity 
which  the  other  person  had  received  from 
my  body  did  then  r«  turn  from  him  to  me 
in  a  sharp  spark,  which  issued  from  his 
hand  F  at  the  very  iiiftaut  that  the  explo- 
sion of  the  prime  conductor  took  place 
upon  the  ball  L. 

44  I  still  continued  upon  tho  insulating 
stool  E,  and  I  dc&ircd  the  other  person  to 
stand  upon  the  floor.  The  returning 
stroke  between  us  was  still  stronger  than 
it  had  yet  been.  The  reason  of  it  was 
this  :  The  other  person  being  no  longer 
insulated,  transmitted  his  superabundant 
electricity  freely  into  the  earth.  I  conse- 
quently became  still  more  negative  than 
before. 

"  Now,    when    the    returning    stroke 


332 


LESSONS  IN  ELECTRICITY. 


came  to  take  place,  not  only  the  elec- 
tricity which  had  passed  from  my  body 
into  the  body  of  the  other  person,  but 
also  the  electricity  which  had  passed 
from  my  body  into  the  earth  (through 
the  other  person),  did  suddenly  return 
upon  inc  from  his  hand  F  to  my 
hand  B,  at  the  same  instant  that  the 
discharge  of  the  prime  conductor  took 
place  upon  the  ball  L.  This  caused  the 
returning  stroke  to  be  stronger  than  be- 
fore." 

Lord  Mahon  fused  metal?,  and  pro- 
duced strong  physiological  effects  by  the 
return  stroke. 

In  nature  disastrous  effects  may  be  pro- 
duced by  the  return  stroke.  The  earth's 
surface,  and  animals  or  men  upon  it,  may 
be  powerfully  influenced  by  one  end  of 
an  electrified  cloud.  Discharge  may  oc- 
cur at  the  other  end,  possibly  miles  away. 
The  restoration  of  the  electric  equilib- 
rium by  the  return  shock  may  be  so 
violent  as  to  cause  death. 

This  was  clearly  seen  and  illustrated  by 
Lord  Mahon.  Fig.  57  is  a  reduced  copy 
of  his  illustration.  JL  B  c  is  the  electri- 
fied cloud,  the  two  ends  of  which,  A  and 
c,  come  near  the  earth.  The  discharge 
occurs  at  c.  A  man  at  F  is  killed  by  the 
returning  stroke,  while  the  people  at  D, 
nearer  to  the  place  of  discharge,  but  far- 
ther from  the  cloud,  are  uninjured. 


TVith  the  viow  of  still  further  testing 
your  knowledge  of  induction,  I  have  here 
copied  a  portion  of  this  admirable  essay  ; 
but  the  entire  memoir  of  Lord  Manor, 
would  constitute  a  most  useful  and  inter- 
esting lesson  in  electricity. 

For  our  own  instruction  we  can  illus- 
trate the  return  shock  thus  : — Connect 
one  arm  of  your  universal  discharger,  fig. 
49,  with  a  conductor  like  c,  fig.  20,  and 
the  other  arm  with  tho  earth.  Bring  c 
within  a  few  inches  of  your  prime  con- 
ductor, but  not  within  striking  distance  ; 
on  working  tho  machine  a  stream  of  fee- 
ble sparks  will  pass  from  point  to  point 
of  the  discharger.  Let  the  prime  con- 
ductor be  discharged  from  time  to  thtoe 
by  an  assistant  ;  at  every  discharge  the 
returning  stroke  is  announced  by  a  flash 
between  the  points  of  the  discharger  at  *. 
If  gun-cotton  with  a  little  fulminating 
powder  scattered  on  it,  or  a  fine  silver 
wire,  be  introduced  between  the  points 
of  tho  discharger,  the  one  is  exploded 
and  the  other  deflagrated. 

The  stream  of  repelled  sparks  first  seem 
may  be  entirely  abolished  by  establishing 
an  imperfect  connection  between  the  con- 
ductor c  and  tho  earth  :  a  chain  resting 
upon  the  dry  table  on  which  the  conduct- 
or stands  will  do.  The  chain  permits 
the  feebler  sparks  to  pass  through  it  in 


Fw.  58. 


LESSORS  IN  ELECTRICITY. 


8*3 


preference  to  crossing  the  space  s  ;  but 
the  returning  stroke  is  too  strong  and 
sudden  to  find  a  sufficiently  open  channel 
through  the  table  and  chain,  and  on  the 
discharge  of  the  prime  conductor  the 
spark  is  seen. 

It  was  tho  action  of  the  return  shock 
upon  a  dead  frog's  limbs,  observed  in  the 
laboratory  of  Professor  Galvani,  that  led 
to  Galvani's  experiments  on  animal  elec- 
tricity ;  and  led  further  to  the  discovery, 
by  Volta,  of  the  electricity  which  bears 
his  name. 

§  32.    The  Leyden  Battery,  its  Currents, 
and  some  of  their  Effects. 

In  the  ordinary  Leyden  battery  de- 
scribed in  §  19  all  the  inner  coatings  are 
connected  together,  and  all  the  outer 
coatisgs  arc  also  connected  together. 
Such  a  battery  acts  as  a  single  large  jar 
of  extraordinary  dimensions. 

Wires  are  warmed  by  a  moderate  elec- 
tric discharge  ;  by  augmenting  the  charge 
they  arc  caused  to  glow  ;  with  a  strength- 
ened charge  the  metal  is  torn  to  pieces  ; 
fusion  follows  ;  and  by  still  stronger 
charges  the  wires  are  reduced  to  metallic 
dust  and  vapor. 

For  such  experiments  the  wire  must  be 
thin.  Without  resistance  we  can  have 
no  heat,  and  when  the  wire  is  thick  we 
have  little  resistance.  The  mechanism 
of  the  discharge,  as  shown  by  the  figures 
produced,  is  different  in  different  wires. 
The  figure  produced  by  the  dust  of  a  def- 
lagrated silver  wire  on  white  paper  is 
shown  in  fig.  58. 

When  the  discharge  of  a  powerful  bat- 
tery is  sent  through  a  long  steel  chain 
with  the  ends  of  its  links  unsoldered,  the 
sparks  between  the  unsoldered  links  carry 
the  incandescent  particles  of  the  steel 
along  with  them.  These  are  consumed 
in  the  air,  a  momentary  blaze  occurring 
along  the  entire  chain.  Chain  cables 
have  been  fused  by  being  made  the  chan- 
nels of  a  flash  of  lightning. 

Retaining  our  conception  of  an  electric 
fluid,  at  this  point  we  naturally  add  to  it 
the  conception  of  a  current.  It  is  the 
electric  current  which  produces  the  effects 
just  described.  In  many  of  our  former 
experiments  we  had  electricity  at  rest 
(static  electricity),  here  we  have  electric- 
ity in  motion  (dynamic  electricity). 
Sending  the  current  from  a  battery 


through  a  fiat  spiral  (the  primary)  formed 
of  fifty  or  sixty  feet  of  copper  wire,  and 
placing  within  a  little  distance  of  it  a 
second  similar  spiral  (the  secondary)  with 
its  ends  connected  ;  the  passage  of  tho 
current  in  the  first  spiral  excites  in  tho 
second  a  current,  \vhich  is  competent  to 
deflagrate  wires,  and  to  produce  all  the 
other5 effects  of  the  electrical  discharge. 
Even  when  the  spirals  are  some  feet  asun- 
der, the  shock  produced  by  the  second- 
ary current  is  still  manifest. 

The  current  from  tbe  secondary  spiral 
may  be  carried  lound  a  third  ;  and  this 
third  spiral  may  be  allowed  to  act  upon 
u  fourth,  exactly  as  the  primary  did  upon 
the  secondary.  A  tertiary  current  is  thus 
evoked  by  the  secondary  in.  the  fourth 
spiral. 

Carrying  this  tcitiary  current  round  a 
fifth  spiral,  and  causiig  it  to  act  induc- 
tively upon  a  sixth,  we  obtain  in  the  lat- 
ter a  current  of  the  fourth  order.  In  this 
way  we  generate  a  long  progeny  of  cur- 
rents, all  of  them  having  the  current  sent 
from  the  battery  through  the  first  spiraJ 
for  a  common  progenitor.  To  Prof. 
Ilenry  of  the  United  States,  and  to  Prof. 
Riess  of  Berlin,  we  are  indebted  for  the 
investigation  of  the  laws  of  these  cur- 
rents. These  researches,  bow  eve*,,  were 
subsequent  to,  and  were  indeed  suggest- 
ed by,  experiments  of  a  similar  character 
previously  made  by  Faraday  with  Voltaic 
electricity. 

Besides  the  electricity  of  friction  and 
induction  we  have  the  following  sources 
and  forms  of  this  power. 

The  contact  of  dissimilar  metals  pro- 
duces electricity. 

The  contact  of  metals  with  liquids  pro- 
duces electricity. 

A  mere  variation  of  the  character  of  the 
contact  of  two  bodies  produces  electricity. 

Chemical  action  produces  a  continuous 
flow  of  electricity  (Voltaic  electricity). 

Heat,  suitably  applied  to  dissimilar 
metals,  produces  a  continuous  flow  of 
electricity  (thermo-electricity). 

The  heating  and  cooling  of  certain 
crystals  produce  electricity  (pyro-electric- 
ity). 

The  motion  of  magnets,  and  of  bodies 
carrying  electric  currents,  produces  elec- 
tricity (magneto-electricity). 

The  friction  of    sand  against  a  metal 


31 


LESSONS  IN  ELECTKICITY. 


plate  produces  electricity. 

The  friction  <-f  condi-nscd  water-parti- 
cles against  a  safety  valve,  or  better  etill 
against  a  box-wood  nozzle  through  which 
Attain  is  driven,  produces  electricity 
(Armstrong's  hydro-c  lectiic  machine). 

These  aro  different  manifestations  of 
one  and  the  same  power  ;  arid  they  are 
all  evoked  by  an  equivalent  expenditure 
of  some  other  power. 

Conclusion. 

Onr  experimental  researches  end  here. 
I  would  now  bespeak  your  attention  for 
five  minutes  longer.  The  cxpcn?iveness 
of  apparatus  is  sometimes  urged  as  an 
obstacle  to  the  introduction  of  science 
into  schools.  I  hope  it  has  been  shown 
that  the  obstacle  13  not  a  real  one.  Leav- 
ing out  of  account  the  few  larger  experi- 
ments, which  have  contributed  but  little 
to  our  knowledge,  it  is  manifest  that  the 
wise  expenditure  of  a  couple  of  guineas 
would  enable  any  competent  teacher  to 
place  the  leading  facts  and  principles  of 
Jrictional  electricity  completely  at  the 
command  of  his  pupils  ;  giving  them 
UiwreDy  precious  knowledge,  arni  still 
more  precious  intellectual  discipline — a 
discipline  which  invokes  observation,  re- 
flection, prevision  by  the  exercise  of  rea- 
son, and  experimental  verification. 

And  here,  if  I  might  venture  to  do  so, 
I  would  urge  upon  the  science  teachers 
of  our  public  and  other  schools  that  the 
immediate  future  of  science  as  a  factor  in 
English  education  depends  mainly  upon 
them.  I  would  respectfully  submit  to 
them  whether  it  would  not  bo  a,  mistake 
to  direct  their  attention  at  present  to  the 
collection  of  costly  apparatus.  Their 
principal  function  just  now  is  to  arouse  a 
lovo  for  scientific  study.  This  is  best 
done  by  the  exhibition  of  the  needful 
faeta  and  principles  with  the  simplest 
possible  appliances,  and  by  bringing  their 
pupils  into  contact  with  actual  experi- 
mental work. 

The  very  time  and  thought  spent  in 
devising  such  simple  instruments  will  give 
tko  teacher  himself  a  grasp  and  mastery 
of  his  subject  which  ho  could  not  other- 
wise obtain  ;  but  it  ought  to  be  known 
by  the  head  meters  of  our  schools  that 
time  is  needed,  not  only  for  devising  such 
instruments,  but  also  for  preparing  tho 
experiment*  to  be  made  with  them  after 


they  have  been  devised.  No  scienco 
teacher  is  f;t  to  meet  his  class  without 
this  distinct  and  special  prenaration  be- 
fore every  lesson.  His  experiments  aro 
part  and  parcel  of  his  language,  and  they 
ought  to  be  as  strict  in  logic,  and  as  free 
from  stammering,  as  his  spoken  words. 
To  rnuko  them  so  may  imply  an  expendi- 
ture of  time  which  few  head  masters  now 
contemplate,  but  it  is  a  necessary  expen- 
diture, and  they  will  act  wisely  in  mak- 
ing provision  for  it. 

To  them,  moreover,  in  words  of 
friendly  warning,  I  would  say,  make 
room  for  science  by  your  own  healthy 
and  spontaneous  action,  and  do  not  wait 
until  it  is  forced  upon  you  by  revolution, 
ary  pressure  from  without.  Tho  condi- 
tion of  things  now  existing  cannot  con- 
tinue. Its  simple  statement  suffices  to 
call  down  upon  it  the  condemnation  of 
every  thoughtful  mind.  With  reference 
10  the  report  of  a  Commission  appointed 
.ast  year  to  inquire  into  the  scientific  in- 
struction of  this  country,  Sir  John  Lub- 
Dock  writes  as  follows  : — "  Tnc  Com- 
missioners have  published  returns  from 
moro  than  a  hundred  and  twenty  of  the 
larijer  endowed  schools.  In  more  th.nn 
half  of  these  no  science  whatever  i» 
taught  ;  only  thirteen  have  a  laboratory, 
and  only  eighteen  possess  any  scientific; 
apparatus.  Out  of  the  whole  number, 
I-j.ss  than  twenty  schools  devote  as  much 
*  >  four  hours  a  week  to  science,  and  only 
thirteen  attach  any  weight  at  all  to  scien- 
tific subjects  in  the  examinations." 

Well  may  the  Commissioners  pronounce 
such  a  state  of  things  to  be  nothing  less 
than  a  national  calamity  !  If  persisted 
in,  it  will  assuredly  be  followed  by  a  re- 
action which  the  truest  friends  of  classi- 
cal culture  in  England  will  have  the  great- 
est reason  to  deplore. 


APPENDIX. 


AN  ELEMENTARY  LECTURE    ON 
MAGNETISM.* 

WE  have  no  reaa.--a  t:>  believe  thsit  the 
sheep  or  tho  dnjj.  or,  indeed,  any  of  the 
bwer  animals,  feel  aa  interest  hi  the 


*  From    the    author's   volume, 
Science.1" 


Fra-jmeate    of 


LESSONS  IN  ELECTRICITY. 


885 


by  \vhich  natural  phenomena  are  regulated. 
A  V  •  I  r.\  iy  I)  •  !(T'i!i'>(l  \>\r  ;i  thunder-storm  ; 
bi.-iU  r.viv  .<:  i  l  >  IO.J-M,  ;irrl  e  t  \->.  return  to 
llioi  stall-*  <ii!rin<:  a*  -1-r  ei  lir^-  ;  bill  neither 
birds  n  >r  can!",  in  f;ir  r-t  wo  know,  ever 
think  of  inquiiing  int  >  tie  causes  of  th^c 
things.  It  is  ntnerwise  with  man.  Tho 
presence  of  natti-al  object?,  the.  occurrence 
of  mtii'-al  event-1,  l!n?va*ied  apnearances  of 
lh".  universe  in  which  he  dwi  1's,  pent  Irate 
beyond  his  organs  of  sense,  and  appeal  to  an 
inner  power  of  which  the  scnsts  aie  the  mere 
instrumrnts  an'l  excitants.  No  fact,  is  to 
him  either  final  or  original.  II  u  cannot  limit 
hinnelf  i)  the  contemplation  of  it  alone,  but 
endeavors  t->  ascertain  its  position  in  a  series 
to  which  the  con.-.tit;iliMn  of  his  mini  assures 
him  il  must  bcl  ing.  H*  icgarels  all  that  he 
witnesses  in  the  present  as  the  efflux  and 
sequence  of  s  m^thina;  that  has  gone  before, 
an  I  a<  the  source  of  a  system  of  events 
whifh  is  to  follow.  The  notion  of  spon- 
tam'ily,  »v  which  ia  his  ruder  state  ho  ao- 
countcd  for  natural  events,  is  abandoned  ; 
the  i'lfu  that  Nature  is  an  aggregate  of  inde- 
pendent pruts  also  disappears,  as  the  connec- 
tion an  1  mr.tunl  dependence  of  physical  pow- 
ers b'v.»mo  mo'e  and  rmre  manifest  ;  until 
h'i  i.->  finally  led.  and  that  chiefly  by  the  sci- 

1  cncf!  of  which  1  happen  this  evening  to  be 
the  exponent,  to  regaul  Nature  as  an  organic 
whole,  as  u  body  each  of  whose  members 
sympathizes  with  the  rest,  changing,  it  is 
true,  from  ages  to  ages,  but  without  one  real 
bieak  '  f  continuity,  or  a  single  interruption 
of  the  fixed  relations  of  cause  and  effect. 

The  system  of  things  which  we  call  Nature 
is,  however,  too  vast  and  various  to  be  studied 
first-han  I  by  any  single  mind.  As  knowledge 
exten  Is  there  is  always  a  tendency  to  sub- 
divide the  fiel.l  of  investigation,  its  various 
paits  being  taken  up  by  different  individuals, 
and  thus  receiving  a  greater  amount  of  atten- 
tion than  could  possibly  be  bestowed  on  them 
if  each  investigator  aimed  at  the  mastery  of 
the  whole.  East,  west,  north,  and  south,  the 
huaian  mind  pushes  its  conquests  ;  but  the 
centripetal  form  in  which  knowledge,  as  a 
whole,  advances,  spreading  evor  wider  on  all 
sides,  is  due  in  reality  to  the  exertions  of  in- 
dividuals, each  of  whom  directs  his  efforts, 
more  or  less,  along  a  single  line.  Accepting, 
in  many  respects,  his  culture  from  his  fellow- 

.  xmro,  taking  it  from  spoken  words  and  from 
•written  books,  in  some  one  direction,  the  stu- 
dent of  nature  mu«t  actually  touch  his  work, 
lie  may  otherwise  be  a  distributor  of  knowl- 
edge, but  not  a  creator,  and  fails  to  attain 
that  vital  it  v  r-f  tli  ought  and  correctness  of 

•  judgment  w7hich  direct  and  habitual  contact 
with  nitiirfil  truth  can  alone  impart. 

One  large  department  of  the  system  of  Na- 
ture which  forms  the  chief  subject  of  my  own 
I'.tudie.s,  and  to  which  it  is  n:/  duty  to  call 
your  attention  this  evening.  '..«.  thai  (.f  PU\MC.-. 
or  natural  philosophy.  This  term  is  "large 

!     uiougtito  cover  tiie  study   of  Nature  gen- 
erally, but  it  is  usually  lestricted  to  a  dcpert 
mem  which,  perhaps,  l.es  closer  to  our  per-, 
tnaa  any  oilier.     It  deals  with  the 


phenomena  and  laws  of  light  and  heat — with 
the  phenomena  and  laws  of  magnetism  and 
electricity — with  those  of  sound — with  the 
pressures  and  motions  of  liquids  and  gases, 
whether  in  a  state  of  translation  or  of  undula- 
tion. The  science  of  mechanics  is  a  portion 
of  natural  philosophy,  though  at  present  so 
large  as  to  ne,ed  the  exclusive  attention  of  him 
who  would  cultivate  it  profoundly.  Astron- 
omy is  the  application  of  physics  to  the  mo- 
tions of  the  heavenly  bodies,  the  vastness  of 
the  field  causing  if,  however,  to  be  regarded 
as  a  department  in  itself.  In  chemistry 
physical  agents  play  important  pa*ts.  By 
heat  and  light  we  cause  bodies  to  combine, 
and  by  heat  and  light  we  decompose  them. 
Electricity  tears  asunder  the  locked  atoms  of 
compounds,  through  their  p^wer  of  separat- 
ing carbonic  acid  into  its  constituents  ;  tine 
solar  beams  build  up  the  whole  vegetable 
world,  and  by  it  the  animal,  v/hile  the"  touch 
of  the  self-same  beams  causcn  hydrogen  and 
chlorine  to  unite  with  sudden  explosion  and 
form  by  their  combinal ion  a  powerful  acid. 
•Thus  physics  and  chemistry  inter  mingle, 
physical  agents  being  empk.yid  by  the  chem- 
ist as  a  means  to  an  ind  ;  while  in  physics 
proper  the  laws  and  phenomena  of  the  agents 
themselves,  both  qualitative  and  quantitative, 
are  the  primary  objects  of  htlention. 

JVly  duty  heie  to-night  is  !o  t-pind  an  hour 
in  telling  how  this*  subject  is  t  >  bj  studied, 
and  how  a  knowledge  of  it  is  lj  bo  imparted 
toothers.  W hen  first  invited  to  do  ihis,  I 
hesitated  before  accepting  the  icsponsibil.iy. 
It  would  be  easy  to  tnteitaiu  you  with  an  ac- 
count of  what  natural  philosophy  hap  accom- 
plished. I  might  point  lo  those  application* 
of  science  regarding  which  we  hear  so  much 
in  the  newspapers, "and  which  we  often  find 
mistaken  for  science  itself.  I  might,  of 
course,  ling  changes  on  the  .steam-engine  and 
the  telegraph,  the  electrotype  and  the  photo- 
graph, the  medical  applications  of  physics, 
and  the  million  other  inlets  by  which  tcieo- 
tific  thought  filters  into  practical  life.  That 
would  be  easy  computed  with  the  task  of  in- 
forming you  how  ycu  arc  to  make  the  study 
of  physics  t  he  instrument  of  your  own  culture, 
how  you  are  to  possess  its  factu  and  make 
them  living  seeds  which  shall  take  io:.t  and 
grow  in  the  mind,  and  not  lie  like  de:ul  lum- 
ber in  the  storehouse  of  memory.  This  is  a 
task  much  heavier  than  the  mere  cataloguing 
of  scientific  achievements  ;  and  it  is  one 
which,  feeling  my  own  w:ini  cf  time  and 
power  to  execute  it  aright,  1  might  well  hesi- 
tate to  accept. 

But  let  me  sink  excuses,  and  attack  the 
work  to  the  best  of  my  ability.  First  and 
foremost,  then,  I  would  advise  you  to  get  a 
knowledge  of  facts  from  actual  oboei  vuii^n. 
Facts  looked  at  directly  are  vital  ;  when  they 
pass  into  words  half  the  sap  is  taken  out  cf 
them.  Ycu  wish,  for  example,  to  get  a 
knowledge  of  magnetism  ;  wul,  provide  your- 
S(  If  with  a  good  bock  on  the  subject,  if  ycu 
can,  but  d)  not  be  content  with  what  the 
book  tells  you  ;  do  not  be  satisfied  with  its 
descriptive  wood-cuts  ;  see  the  actval  thing 


306 


LESSORS  IN  ELECTRICITY. 


yourstlf.  Half  of  our  book-writers  describe 
experiments  which  they  never  made,  and 
Uieir  descriptions  often  luck  both  force  and 
truth  ;  but  no  matter  how  clever  or  conscien- 
tious they  may  be,  their  written  words  cannot 
supply  the  place  of  actual  observation. 
Every  fact  has  numerous  radiations,  which 
are  shorn  off  by  the  man  who  describes  it. 
Go,  then,  to  a  philosophical  instrumeut- 
maker,  and  give,  according  to  your  means, 
for  a  straight  bar-magnet  say,  half  a  crown, 
or,  if  you  can  afford  it,  five  shillings  for  a 
pair  of  thorn  ;  or  get  a  smith  to  cut  a  length 
of  ten  inches  from  a  bar  of  steel  an  inch  wide 
and  half  an  inch  thick  ;  rile  its  ends  decently, 
harden  it,  and  get  somebody  like  myself  to 
magnetize  it.  Two  bar-magnets  are  better 
than  one.  Procure  some  darning-needles 
such  as  these.  Provide  yourself  also  with  a 
little  unspun  silk  ;  which  will  give  you  a  sus- 
pending tibre  void  of  torsion  ;  make  a  little 
loop  of  paper  or  of  wire,  thus,  and  attach 
your  fibre  to  it.  Do  it  neatly.  In  the  loop 
place  your  darning-needle,  and  bring  the  two 
ends  or  poles,  as  they  are  called,  of  your 
magnet  successively  up  to  cither  end  of  the 
needle.  Both  the  poles,  you  find,  attract  both 
ends  of  the  needle.  Replace  the  needle  by  a 
bit  of  annealed  iron  wire,  the  same  effects  en- 
su<\  Suspend  successively  little  rods  of  lead, 
copper,  silver,  or  brass,  of  wood,  glass, 
ivory,  or  whalebone  ;  the  magnet  produces 
no  sensible  effect  upon  any  of  these  suh- 
itances.  You  thence  infer  a  special  property 
in  the  case  ot  steel  and  iron.  Multiply  your 
experiments,  however,  nndyou  will  find  that 
some  other  substances  besides  iron  are  acted 
upon  by  your  magnet.  A  rod  of  the  metal 
nickel,  or  of  the  metal  cobalt,  from  which  the 
blue  color  used  by  painters  is  derived,  ex- 
kibits  powers  similar  to  thoee  observed  with 
tho  iron  and  steel. 

In  studying  the  character  of  the  force  you 
may,  howeve^r,  confine  yourself  to  iron  and 
steel,  which  are  always  at  hand.  Make  your 
experiments  with  the  darning-necdlo  over 
and  over  again  ;  operate  on  both  ends  of  tho 
needle  ;  try  both  ends  of  the  magnet.  Do 
not  think  the  work  stupid  ;  you  are  convers- 
ing with  Nature,  and  must  acquire  a  certain 
grace  and  mastery  over  her  language  ;  and 
these  practice  can  alone  impart.  Let  every 
movement  be  made  with  care,-  and  -  avoid 
slovenliness  from  the  outset.  In  every  one 
of  your  experiments  endeavor  to  feel  the  re- 
sponsibility of  a  moral  agent.  Experiment, 
as  I  have  said,  is  the  language  by  which  we 
address  Nature,  and  through  which  she  sends 
her  replies  ;  in  the  use  cf  this  language  a 
lack  of  straightforwardness  is  as  possible  and 
as  prejudicial  as  in  the  spoken  language  of 
the  tongue.  If  you  w  ish  to  become  acquaint- 
ed with  the  truth  of  Nature,  you  must  from 
the  first  resolve  to  deal  with  her  sincerely. 

Now  remove  your  iicedlefrorn  its  loop,  and 
draw  it  from  end  to  end  along  one  of  the 
ends  of  thomngnet ;  re-suspend'it,  tnd  repeat 
your  former  experiment.  You  fliul  the  result 
different.  You  now  find  that  each  extremity 
•f  the  magnet  attract*  one  end  of  the  needle 


nnrt  repels  the  other.  The  simple  attraction 
observed  in  the  first  instance  is  now  replaced 
by  a  dual  force.  Repeat  the  experiment  till 
you  have  thoroughly  observed  the  ends  which 
attract  and  those  which  repel  each  other. 

Withdraw  the  magnet  entirely  from  tho 
vicinity  of  your  needle,  and  leave  the  loUer 
freely  suspended  by  its  fibre.  Shelter  ii  as 
well  as  you  can  from  currents  of  air,  and  if 
you  have  iron  -buttons  on  you-r  coa't  or  a  slee? 
penknife  in  your  pocket,  beware  of  their  ac 
tion.  If  you  work  at  iiight,  beware  of  iron 
candlesticks,  or  of  brass  ones  with  .iron  rod.-} 
inside.  Freed  from  such  disturbaaces,  the 
needle  takes  up  a  certain  determinate  po- 
sition. It  sets  its  length  nearly  north  an'f 
south.  Draw  it  aside  from  this  position  and 
let  it  go.  After  several  oscillations  it  will 
again  come  to  it.  If  you  have  obtained  your 
magnet  from  a  philosophical  instrument- 
maker,  you  will  see  a  mark  on  one  of  its  emls. 
Supposing,  then,  that  you  drew  your  needle 
along  the  end  thus  marked,  and  that  the  eye- 
eud  of  your  needle  was  the  last  to  quit  the 
magnet,  you  will  find  that  the  eye  turns  to 
the  south,  the  point  of  the  needle  turning 
toward  the  north.  Make  suie  of  this,  and  do 
not  take  this  statement  on  my  authority. 

Now  lake  a  second  darning-needle  like  the 
first,  and  magnetize  it  in  precisely  the  same 
manner  :  freely  suspended  it  also  will  turn  its 
point  to  the  north  and  its  eye  to  the  south. 
Your  next  step  is  to  examine  the  action  ot"  the 
two  needles  which  you  have  thus  magnelizvri 
upon  each  other. 

Take  one  ot  them  in  your  hand,  and  leave 
the  other  suspended  ;  bring  the  eye-end  of 
the  former  near  the  eye-end  of  the  latter  ;  the 
suspended  needle  retreats  :  it  is  repelled. 
Make  the  same  experiment  with  the  two 
points,  you  obtain  the  same  result,  the  sus- 
pended needle  is  repelled.  Now  cause  tho 
dissimilar  ends  to  acton  each  other  —  you 
have  attraction — point  attracts  eye  and  eye 
attracts  point.  Prove  the  reciprocity  of  this 
action  by  removing  the  suspended  needle, 
and  putting  the  other  in  its  place.  You  ob- 
tain the  same  result.  The  attraction,  then, 
is  mutual,  and  the  repulsion  is  mutual,  and 
you  have  thus  demonstrated  in  the  clearest 
manner  the  fundamental  law  of  magnetism, 
that  like  poles  repel,  and  that, .unlike  poles  at- 
tract each  other.  You  may  say  that  this  is 
all  easily  understood  without  doing  ;  but  do 
it,  and  your  knowledge  will  not  be  confined 
to  what  I  have  uttered  here. 

I  have  said  that  one  end  of  your  magnet 
has  a  mark  upon  it ;  lay  several  silk  fibres 
together,  sa  as  to  get  sufficient  strength,  or 
employ  a  thin  silk  ribbon,  and  form  a  loop 
large  enough  to  hold  your  magnet.  Suspend 
it ;  it  turns  its  marked  end  toward  the  north. 
This  marked  end  is  that  which  in  England  is 
called  the  north  pole.  If  a  common  smith 
has  made  your  magnet,  it  will  be  convenient 
to  determine  its  noith  pole  yourself,  and  to 
mark  it  with  a  file.  You  vary  your  experi- 
ments by  causing  your  magneti/ed  darning- 
needle  to  attract  and  repel  your  larpe  mag- 
net;  it  is  guile  competent  to  do  so.  In 


LESSONS  IN  ELECTRICITY. 


337 


netlzlng  Iho  needle  I  hare  supposed  the  rye- 
end  to  be  the  last  to  quit  tho  marked  end  of 
tho  magnet  ;  that  end  of  th.'  needle  is  a  south 
pole.  TJie  end  which  la  4  quits  the  magnet 
is  always  opposed  in  p'-larity  to  the  end  of 
I  ho  magnet  with  which  it  lias  been  in  contact. 
BiMuglit  near  each  other  they  mutually  at- 
iract,  and  thus  demons. rate  that  they  ate  uu- 
liki1  p  >!cs. 

You  may  perhaps  learn  all  this  in  a  single 
hour;  but  spend  seveial  at  it,  if  necessary; 
an  1  remember,  understanding  it  is  not  sulli- 
cient  :  you  mu^t  obtain  a  manual  aptitude  in 
addressing  Nature.  If  you  speak  to  your  fel- 
low-man, you  are  not  entitled  to  use  jargon. 
Bad  experiments  nre  juigon  addressed  to  Na- 
ture, and  just  as  much  tj  be  deprecated.  A 
manual  dexterity  in  ill  astral  ing  the  interac- 
titin  of  magnetic  poles  is  of  the  utmost  impor- 
tance at  tuis  stage  of  your  progress,  and  you 
must  not  neglect  attaining  this  power  over 
your  implements.  As  yru  proceed,  more- 
over, you  will  be  tempted  to  do  more  than  I 
can  possibly  suggest.  Thoughts  will  cccur 
to  you  which  you  will  endeavor  to  follow 
out  ;  questions  will  arise  which  you  will  try 
to  answer.  The  same  experiment  may  be 
twenty  things  to  twenty  people.  Having  wit- 
nessed the  action  of  pole  on  pole  through  the 
»lr,  you  will  perhaps  try  whether  tbe  mng- 
pctic  power  is  not  to  be  screened  off.  You 
use  plates  of  glass,  wood,  slate,  pasteboaid, 
or  gutta-percha,  but  find  them  all  pervious  to 
this  wondrous  force.  One  magnetic  pole  nets 
upon  another  through  these  bodies  as  if  they 
were  not  present.  And  should  you  become 
a  patentee  for  the  regulation  of  fillips'  com- 
passes, you  will  not  fall,  as  some  projectors 
have  done,  into  the  error  of  screening  off  the 
magnetism  of  the  ship  by  the  inter  position  of 
auch  substances. 

If  you  wish  to  tench  a  class  you  must  con- 
trive that  the  effects  which  you  have  thus  far 
witnessed  for  yourself  shall  be  witnessed  by 
twenty  or  thirty  pupils.  And  here  your  pri- 
vate ingenuity  must  come  into  play.  You 
will  attach  bits  of  paper  to  your  needles,  so 
as  to  render  their  movements  visible  ot  a  dis- 
tance, denoting  the  north  and  south  poles  by 
different  colors,  say  gieen  and  red.  \ou 
may  also  improve  upon  your  darning-needle. 
Take  a  strip  of  sheet-steel,  the  rib  of  a  lady's 
Hays  will  answer,  heat  it  to  vivid  redness  and 
plunge  it  into  cold  water.  It  is  thereby  hard- 
ened— rendered,  in  tact,  almost  as  brittle  as 
glass.  Six  inches  of  this,  magnetized  in  the 
manner  of  the  darning-needle,  will  be  better 
aole  to  carry  your  paper  indexes.  Having 
aeriireil  such  a  strip,  you  proceed  thus  : 

Magnetize  a  small  sewing-needle  and  deter- 
mine its  [>oles  ;  or,  break  half  an  inch  or  an 
inch  off  your  magnetized  darning-needle,  and 
suspend  it  by  a  tine  silk  fibre.  The  sewing- 
needle  or  the  fiagment  of  the  darning-needle 
is  now  to  be  used  as  a  test-needle  to  examine 
the  distribution  of  the  magnetism  in  your 
strip  of  steel.  Hold  the  strip  upright  in  your 
left  hand,  and  cause  the  test-needle  to  ap- 
proach the  lower  end  of  your  strip  ;  one  end 
is  attracted  the  other  ie  repelled.  Raise  your 


needle  along  the  strip  ;  its  oscillations,  which 
at  first  weie  quick,  become  slower  ;  opposite 
the  middle  of  the  strip  they  cease  enli  ely  ; 
neither  end  of  the  needle  is  attracted  ;  above 
the  middle  the  test-needle  turns  suddenly 
round,  its  other  end  being  now  attiacted.  Go 
through  the  experiment  thoroughly  ;  yen 
thus  learn  that  the  entire  lower  half  cf'the 
strip  Attracts  one  end  of  the  needle,  while  the 
entire  upper  half  attracts  the  opposite  end. 
Supposing  the  north  end  of  your  little  reed-lc 
to  be  that  attracted  below,  you  infer  that  tho 
entire  Ijwer  half  of  your  magnetized  strip 
exhibits  south  magnetism,  while  the  enliio 
upper  half  exhibits  north  magnetism.  So 
far,  then,  you  have  determined  the  distribu- 
tion of  magnetism  in  your  strip  of  steel. 

You  look  HI  this  fact,  you  think  of  it  ;  in 
its  suggest iveness  the  value  of  the  i  xpc-i  imeut 
chiefly  consists.  The  thought  arises,  "  What 
will  occur  if  1  break  my  ship  of  sled  ncm&s 
iu  the  middle  ?  Shall  1  obtain  two  magnet*,, 
each  possessing  a  single  pole  ?"  Try  the  ex- 
periment ;  break  your  sliip  of  steel,  and  le&t 
each  half  as  you  tested  the  whole.  The  mere- 
presentation  of  its  two  ends  iu  succession  tu. 
your  test-needle  suffices  to  show  you  that 
you  have  not  a  magnet  with  a  single  pole,, 
that  each  half  possesses  two  poles  with  aneiK 
tral  point  between  them.  And  if  you  again? 
break  the  half  into  two  other  halves,  you  will 
find  that  each  quaiter  of  the  original  (strip  ex.- 
hibits  precisely  Ihe  sanio  magnetic  dlsliibu-- 
lion  as  the  strip  itself.  You  limy  continue  th* 
breaking  process  ;  no  matter  how  small  your 
fragment  may  be,  it  still  possesses  two  op-. 
posite  poles  a"nd  a.  neutral  point  between  them. 
Well,  your  hand  ceases  to  break  vUieie  hi  cak- 
ing becomes  a  mechanical  impossibility  :  but 
does  the  mind  stop  there  ?  No  :  you  follow 
the  breaking  process  in  idea  when  you  can  no 
longer  icalize  it  in  fact  ;  your  thoughts  wan- 
der amid  the  very  atoms  of  your  steel,  and 
you  conclude  that  tach  atom  is  a  magnet, 
and  that  the  force  exerted  by  the  strip  of  steel 
is  the  mere  summation  or  lesullant  of  the 
forces  of  its  ultimate  particles. 

Here,  then,  is  an  exhibition  of  power  which 
we  can  call  forth  cr  cause  to  disappear  at 
pleasure.  We  magnetize  onrstiip  ot  steel  by 
drawing  it  along  the  pole  of  a  magnet  ;  we 
can  demagnetize  it,  or  reverse  its  magnetism, 
by  properly  drawing  it  along  the  same  pole- 
in  the  opposite  direction.  What,  then,  i-  the 
real  nature  of  this  wondrous  change  ?  What 
is  it  that  takes  place  among  the  atoms  of  the 
steel  when  thefcubstance  is  magnetized  ?  The- 
question  leads  us  beyond  the  legion  of  sense, 
and  into  that  of  imagination.  This  faculty, 
indeed,  is  the  divining  iod  of  the  man  of  sci- 
ence. Not,  however,  an  imagination  which 
catches  its  creations  fn,rn  the  air.  but  one  in- 
formed and  inspired  by  fj'Cts,  caj  able  </f  seiz- 
ing fiimly  on  a  physical  image  as  a  principle, 
of  discerning  its  consequences,  and  of  ('evising 
means  whereby  these  t\  recasts  of  thought  may 
be  brought  to  an  experimental  test.  If  Mich 
a  principle  be  adequate  to  account  for  all  the 
phenomena,  if  from  an  assumed  cause  the  ob- 
served facts  necessarily  follow,  wecail  the  as 


33S 


LESSONS  IN  ELECTRICITY. 


sumption  a  theory,  and,  once  possessing  it,we 
can  not  only  revive  at  pleasure  facts  already 
known,  but  we  can  predict  ot tiers  which  we 
luwe  never  seen.  Thus,  tnen,  in  the  prose- 
cut  ir/n  of  physical  science,  our  powers  of  ob 
serration,  memory,  imagination,  and  in- 
teren-e,  are  nil  drawn  upon.  We  observe 
Jncts  and  i-tore  them  tip  ;  imagination  breed6' 
upon  these  memoiies,  end  by  the  aid  of 
leasou  tries  to  discern  their  interdependence. 
The  theoretic  principle  flashes,  or  slovly 
dawns  upon  the  mind,  aud  then  the  deductive 
faculty  interposes  to  carry  out  the  principle 
to  its  logical  consequences,  A  perfect  theory 
gives  dominion  over  natural  facts  ;  and  even 
an  assumption  which  can  only  partially  stand 
the  test  of  a  comparison  with  facts,  may  be 
of  eminent  11*0  in  enabling  us  tq  connect  and 
classify  groups  of  phenomena.  The  theory 
of  magnetic  fluids  is  of  this  latter  character, 
and  with  it  we  must  now  make  ourselves 
familiar. 

With  the  view  of  stamping  the  thing  more 
firmly  on  your  minds,  1  will  make  use  of  a 
strong  and  vivid  image.  In  optics,  red  aud 
green  are  called  complementary  colors  ;  their 
mixture  produces  white.  Now  I  ask  you  to 
|  imagine  each  of  these  colors  to  possess  a  self- 
\  repulsive  power  ;  that  red  repels  red,  and 
*  that  greeti  repels  green  ;  but  that  red  attracts 
green  and  green  attracts  red,  the  attraction  of 
5  the  dissimilar  colors  being  equal  to  the  repul- 
sion of  the  similar  ones.  Imagine  the  two 
colors  mixed  so  as  to  produce  white,  and  sup- 
pose two  strips  of  wood  painted  with  this 
white  ;  what  will  be  their  action  upon  each 
othei  ?  Suspend  one  of  them  freely  as  we 
suspended  our  darning-needle,  and  bring  the 
other  near  it;  what  will  occur?  The  red 
component  of  the  strip  you  hold  in  your  hand 
will  *epel  the  red  component  of  your  sus- 
pended strip,  but  then  it  will  attract  the 
green  ;  and  the  forces  being  equal  they  neu- 
tralize each  other.  In  fact,  the  least  reflec- 
tion shows  you  that  the  strips  will  be  as  in- 
different to  each  other  as  two  unmagnetized 
darning-needles  would  be  under  the  same  cir- 
cumstances. 

But  suppose,  instead  of  mixing  the  colors, 
w*i  painted  one  half  of  each  strip  from  centre 
to  end  red,  and  the  other  half  green,  it  is  per- 
fectly manifest  that  the  two  strips  would  now 
behave  toward  each  other  exactly  as  our  two 
magnetized  darning  -  needles— the  red  end 
would  repel  the  red  and  attract  the  green, 
the  green  would  repel  the  green  and  attract 
the  red  ;  so  that,  assuming  twro  colors  thus 
related  to  each  other,  we  could  by  their  mix- 
ture produce  the  neutrality  of  an  unmag- 
netized body,  while  by  their  separation  we 
could  produce  the  duality  of  action  of  mag- 
netized bodies. 

But  you  have  already  anticipated  a  defect 
in  my  conception  ;  for  if  we  break  one  of  our 
strips  of  wood  in  the  middle  we  have  one  half 
entirely  red  and  the  other  entirely  green,  and 
with  these  it  would  be  impossible  to  imitate 
the  action  of  our  broken  magnet.  How,  then, 
must  we  modify  our  conception  ?  We  must 
evidently  suppose  each  atom  of  wood  painted 


green  on  one  face  and  red  on  the  opposite? 
one.  If  this  were  done  the  resultant  action 
of  all  the  atoms  would  exactly  resemble  tho 
action  of  a  magnet.  Here,  also,  if  the  two 
opposite  colois  of  each  atom  could  be  caused, 
to  mix  so  as  to  produce  white,  we  should 
have,  as  before,  pei feet  neutrality. 

Substitute  in  your  minds  far  these  two  self 
repeilant  anl  mutually  attractive  colors  two 
invisible  self-repellant  and  mutually  attrac- 
tive fluids,  which  in  ordinary  steel  are  mixed 
to  form  a  neutral  compound,  but  which  tho 
act  of  magnetization  separnl.es  from  each 
other,  placing  the  opposite  fluids  on  the  op- 
posite faces  of  each  atom,  and  you  have  a 
perfectly  distinct  conception  of  the  celebrated 
theory  of  magnetic  fluids.  The  strength  of 
the  magnetism  excited  is  supposed  to  be  pro- 
portional to  the  quantity  of  neutral  fluid  de- 
composed. According  to  this  theory  nothing 
is  actually  transferred  from  the  exciting  mag 
net  to  the  excited  steel.  The  act  of  mag- 
netization consists  in  the  forcible  separation 
of  two  powers  which  existed  in  the  steel  be- 
fore it  was  magnetizefl,  but  which  then  neu- 
tralized each  other  by  their  coalescence.  And 
i  f  you  lest  your  magnet  after  it  has  excited  a 
hundred  pieces  of  steel,  you  will  find  that  it 
has  lost  no  force — no  more,  indeed,  th>m  I 
should  lose  had  my  words  such  a  magnetic  in- 
fluence on  your  minds  as  to  excite  m  them  p. 
strong  resolve  to  study  natural  philosophy. 
I  should,  in  fact,  be  the  gainer  by  my  own 
utterance  and  by  the  reaction  of  your 
strength  ;  and  so  also  the  magnet  is  the  gainer 
by  the  reaction  of  the  body  which  it  mag- 
netizes. 

Look  now  to  your  excited  piece  of  steel ; 
figure  each  atom  to  your  minds  with  its  op- 
posed fluids  spread  over  its  opposite  faces. 
How  can  this  state  of  things  be  permanent  ? 
The  fluids,  by  hypothesis,  attract  each  other  : 
what,  then,  keeps  them  apart  ?  Why  do  they 
not  instantly  rush  together  across  the  equator 
of  the  atom,  aud  thus  neutralize  each  other? 
To  meet  this  question,  philosophers  have 
been  obliged  to  infertile  existence  of  a  special 
force  which  holds  the  fluids  asunder.  They 
call  it  coercive  force  ;  and  it  is  found  that  those 
kinds  of  steel  which  offer  most  resistance  to 
being  magnetized,  which  require  the  greatest 
amount  of  coercion  to  tear  their  fluids  asunder, 
are  the  very  ones  which  offer  the  greatest  re- 
sistance to  the  reunion  of  the  fluids  after  they 
have  been  once  separated.  Such  kinds  of 
steel  are  most  suited  to  the  formation  of 
permanent  magnets.  It  is  manifest,  indeed, 
that  without  coercive  force  a  permanent  mag- 
net would  not  be  at  all  possible. 

You  have  not  forgotten  that,  previous  to 
magnetizing  your  darning-needle,  botfi  its 
ends  were  attracted  by  your  magnet  ;  and 
that  both  ends  of  your  bit  of  iron  wire  were 
acted  upon  in  the  same  way.  Probably  also 
long  before  this  you  will  have  dipped  the  end 
of  your  magnet  among  iron  filings,  and  ob- 
served how  they  cling  to  it,  or  into  a  nail- 
box,  and  found  how  it  drags  the  nails  after 
it.  I  know  very  well  that  Vf  you  are  not  the 
slaves  of  routine,  you  will  have  by  this  time 


LESSONS  IN  ELECTRICITY. 


380 


Jone  many  things  that  I  have  not  told  you  to 
do,  and  thus  multiplied  your  experience  be- 
yond what  1  have  indicated.  You  ate  almost 
bure  to  have  caused  a  bit  of  iron  to  hang  from 
tkt»  end  of  your  magnet,  and  you  hare  prob- 
ably succeeded  in  causing  a  second  piece  to 
attach  itself  to  the  first,  a  third  to  the  second  ; 
until  finally  the  force  lias  become  too  feeble 
to  bear  the  weight  of  more.  If  you  have 
operated  with  nails,  you  may  have  observed 
that  the  points  and  edges  hold  together  with 
the  greatest  tenacity  ;  and  that  u  bit  of  iron 
clings  more  firmly  to  the  corner  of  your  raag- 
net  than  to  one  of  its  flat  surfaces.  In  short, 
you  will,  in  all  likelihood,  have  enriched  your 
experience  in  many  ways  without  any  special 
direction  from  me. 

Well,  the  magnet  attracts  the  nail,  and  that 
nail  attracts  a  second  one.  This  proves  tiiat 
the  nail  in  contact  willi  the  magnet  has  had 
the  magnetic  quality  developed  m  it  by  that 
contact.  If  it  be  withdrawn  from  the  mag- 
net, its  power  to  attract  its  fellow-nail  ceases. 
Contact,  however,  is  not  necessary.  A  sheet 
of  glass  or  paper,  or  a  space  of  air,  may  exist 
between  the  magnet  and  the  nail  ;  the  latter 
is  still  magnetized,  though  uot  so  forcibly  as 
when  in  actual  contact.  The  nail  then  pre- 
sented to  the  magnet  is  itself  a  temporary 
magnet.  That  end  which  is  turned  toward 
the 'magnetic  pole  has  the  opposite  magnetism 
of  the  pole  which  excites  it  ;  the  end  most 
remote  from  the  pole  has  the  same  magnet- 
ism as  the  pole  itself,  and  between  the  two 
poles  the  nail,  like  the  magnet,  possesses  a 
magnetic  equator. 

Conversant  a.'?  you  now  are  with  the  theory 
of  magnetic  fluids,  you  have  already,  I  doubt 
not,  anticipated  me  in  imagining  the  exact 
condition  of  the  iron  under  the  influence  of 
the  magnet.  You  picture  the  iron  as  possess- 
ing the  neutral  fluid  in  abundance  ;  you  pic- 
ture the  magnetic  pole,  when  brought  near, 
decomposing  the  fluid  ;  repelling  the  fluid  of 
a  like  kind  with  itself,  and  attracting  the  un- 
like fluid  ;  thus  exciting  in  the  parts  of  the 
iron  nearest  to  itself  the  opposite  polarity. 
But  the  iron  is  incapable  of  becoming  a  per- 
manent magnet.  It  only  shows  its  virtue  as 
long  as  the  magnet  acts  upon  it.  What,  then, 
does  the  iron  lack  which  the  sluel  possesses? 
It  lacks  coercive  force.  Its  fluids  are  sepa- 
rated with  ease,  but,  once  the  separating 
cause  is  removed,  they  flow  together  again, 
and  neutrality  is  restored.  Y  our  imagination 
must  be  quite  nimble  in  picturing  these 
changes.  You  must  be  able  to  see  the  fluids 
dividing  and  reuniting  according  as  the  mag- 
net is  brought  near  or  withdrawn.  Fixing  a 
definite  pole  in  your  imagination,  you  must 
picture  the  precise  arrangement  of  the  two 
fluids  with  reference  to  this  pole.  And  you 
nust  not  only  be  well  drilled  in  the  use  of 
this  mental  imagery  yourself,  but  you  must 
bo- able  fo  arouee  the  same  pictures  in  the 
minds  of  your  pupils,  and  satisfy  yourself 
that  they  possess  this  power  of  placing  iiciu- 
ally  before  themselves  magnets  and  iron  in 
various  positions,  and  describing  the  exact 
magnetic  slate  of  the  iron,  in  each  j/ailicultir 


case.  The  mere  facts  of  magnetism  will  have 
their  interest  immensely  augmented  by  an 
acquaintance  with  those  hidden  principles 
whereon  the  facts  depend.  Still,  while  you 
use  this  theory  of  magnetic  fluids  to  track  out 
the  phenomena  and  link  them  together,  bo 
sure  to  tell  your  pupils  that  it  is  to  be  regarded 
as  a  symbol  merely — a  symbol,  moreover, 
which  is  incompetent  to  cover  all  the  facts,* 
but  which  does  good  practical  service  while 
we  arc  waiting  for  the  actual  truth. 

This  state  of  excitement  into  which  the  soft 
iron  is  thrown  by  the  influence  of  the  magnet, 
is  sometimes  called  "magnetization  by  in- 
fluence." More  commonly,  however,  the 
magnetism  is  said  to  bo  "induced"  in  the 
soft  iron,  and  hence  this  way  of  magnetizing 
is  called  "  magnetic  induction."  Now,  there 
is  nothing  theoretically  perfect  in  Nature  : 
there  is  no  iron  so  soft  as  not  to  possess  a 
certain  amount  of  coercive  force,  and  no  steel 
so  hard  as  not  to  be  capable,  in  some  degree, 
of  magnetic  induction.  The  quality  of  steel 
is  in  some  measure  possessed  by  iron,  and  the 
quality  of  iron  is  shared  in  some  degree  by 
steel.  It  is  in  virtue  of  this  latter  fact  that  the 
unmagnetiztd  darning  needle  was  attracted 
in  your  first  experiment ;  and  from  this  you 
may  at  once  deduce  the  consequence  that, 
after  the  steel  has  been  magnetized,  the  ;e- 
pulsive  action  of  a  magnet  must  be  always 
less  than  its  attractive  action.  For  the  re- 
pulsion is  opposed  by  the  inductive  action  of 
the  magnet  on  the  steel,  while  the  attraction 
is  assisted  by  the  same  inductive  action. 
Make  this  clear  to  your  miuds,  and  verify  it 
by  your  experiments.  In  sume  cases  you  can 
actually  make  the  attraction  due  to  the  tem- 
porary magnetism  overbalance  the  repulsion 
due  to  the  permanent  magnetism,  and  thus 
cause  two  poles  of  the  Fame  kind  apparently 
to  attract  each  other.  When,  however,  good 
hard  magnets  act  on  each  other  from  a  suffi- 
cient distance,  the  inductive  action  practi- 
cally vanishes,  and  the  repulsion  of  like  poles 
is  sensibly  equal  to  the-  attraction  of  unlike 
ones. 

I  dwell  thus  longon  elementary  principles, 
because  they  are  of  the  first  importance,  and 
it  is  the  temptation  of  this  age  of  unhealthy 
cramming  to  neglect  them.  Now  follow  me 
a  little  further.  In  examining  the  distribu- 
tion of  magnetism  in  your  strip  of  steel,  you 
raised  the  needle  slowly  from  bottom  to  top, 
and  found  what  we  called  a  neutral  point  at 
the  centre.  Now  does  the  magntt  really  ex- 
ert no  influence  on  the  pole  presented  to  its 
centre  ?  Let  us  pee. 

Let  S  N,  Fig.  1,  be  your  magnet,  and  let  i 
n  represent  a  particle  of  north  magnetism 
placed  exactly  opposite  the  middJe  of  the 
magnet.  Of  course  this  is  an  imaginary  case, 
as  you  can  never  in  reality  thus  detach  your 
north  magnetism  from  its  neighbor.  What  is 

*  This  theory  breaks  down  when  applied  to  diamag- 
netic  bodies,  which  are  repelled  by  magnets.  Like 
soft  iron,  fnch  bodies  nre  thrown  into  a  state  of  tem- 
porary excitement  in  virtue  of  which  they  are  repelled, 
bur  any  attempt  10  explain  wirh.  a  repulsion  by  the  de- 
composition of  a  fluid  -will  demonstrate  its  own 
futility. 


S40 


LESSONS  IN  ELECTRICITY. 


the  action  of  the  two  poles  of  the  magnet 
on  n  ?  Your  reply  will  of  course  be  that  the 
pole  S  attracts  n  while  the  pole  N  repels  it. 
Let  the  magnitude  and  direction  of  the  at- 
traction be  expressed  by  the  line  n  m,  and  the 
magnitude  and  direction  of  the  repulsion  by 
the  line  n  o.  Now  the  particle  n  being  equally 
distant  from  S  and  N,  the  line  no,  expressing 
the  repulsion,  will  be  equal  to  m  n,  which 
expresses  the  attraction,  and  the  particle  nt 
acted  upon  by  two  such  force-s,  must  evi- 
dently move  in  the  direction^?  n,  exactly  mid- 
way between  in  n  and  n  o.  Hence  you  see 
that,  although  there  is  no  tendency  of  the 
particle  n  to  move  toward  the  magnetic 
equator,  there  is  a  tendency  on  its  part  to 
move  parallel  to  the  magnet.  If  instead  of  a 
particle  of  north  magnetism  we  placed  a  par- 
ticle of  south  magnetism  opposite  to  the  mag- 


FIG.  1. 

netic  equator,  it  would  evidently  be  urged 
along  the  line  n  q  ;  and  if  instead  of  two  sep- 
arate particles  of  magnetism  we  place  H  little 
magnetic  needle,  containing  both  north  and 
south  magnetism,  opposite  the  magnetic 
equator,  its  south  pole  being  urged  along  n  q, 
and  its  north  along  n  p,  the  little  needle  will 
be  compelled  to  set  itself  parallel  to  the  mag- 
net S  N.  Make  the  experiment,  and  satisfy 
yourselves  that  this  is  the  case. 

Substitute  for  your  magnetic  needle  a  bit 
of  iron  wire,  devoid  of  permanent  magnetism, 
and  it  will  set  itself  exactly  as  the  needle 
does.  Acted  upon  by  the  magnet,  the  wire, 
as  you  know,  becomes  a  magnet  and  behaves 
as  such  ;  it  will,  of  course,  turn  its  north  pole 
to  ward  p,  and  south  pole  toward  q,  just  like 
the  needle. 

But  supposing  you  shift  the  position  of 
your  particle  of  north  magnetism,  and  bring 
it  nearer  to  one  end  of  your  magnet,  than  to 
the  other,  the  forces  acting  on  the  particle 
are  no  longer  equal ;  the  nearest  pole  of  the 
magnet  will  act  more  powerfully  on  the  par- 


Fio.  2. 


tide  than  the  more  distant  one.  Let  S  N, 
Fig.  2,  be  the  magnet  and  n  the  particle  of 
north  magnetism  in  its  new  position.  Well, 
it  is  repelled  by  N,  and  attracted  by  S.  Let 
the  repulsion  be  represented  in  magnitude 
and  direction  by  the  line  n  o,  and  the  attrac- 
tion by  the  shorter  line  n  in.  The  resultant 
of  these  two  forces  will  bo  found  by  complet- 
ing the  parallelogram  m  n  o  p,  and  drawing 
its  diagonal  n  p.  Along  np,  Ihrn,  a  particle 
of  north  magnetism  would  be  urged  by  the 
simultaneous  action  of  S  and  N.  Substitut- 
ing a  particle  of  south  magnetism  for  n,  the 
same  reasoning  would  lead  to  the  conclusion 
that  the  particle  would  be  urged  along  n  q, 
and  if  we  place  at  n  a  short  magnetic  needle, 
its  north  pole  will  be  urged  along  n  p,  its 
south  pole  along  n  q,  9nd"the  onlv  ^osition 
possible  to  the  needle,  thus  acted  on,  is  along 
the  line  p  q,  which,  as  you  see,  is  no  longer 
parallel  to  the  magnet.  Verify  this  by  actual 
experiment. 

In  this  way  we  might  go  round  the  entire 
magnet,  and  considering  its  two  poles  as  two 
centres  from  which  the  force  emanates,  we 
could,  in  accordance  with  ordinary  mechani- 
cal principles,  assign  a  definite  direction  to 
the  magnetic  needle  at  every  particular  place. 
And  substituting,  as  before,  a  bit  of  iron 
wire  for  the  magnetic  needle,  the  positions  of 
both  will  be  the  same. 

Now,  I  think,  without  further  preface,  you 
will  be  able  to  comprehend  for  yourselves, 
and  explain  to  others,  one  of  the  most  in- 
teresting effects  in  the  whole  domain  of  mag- 
netism. Iron  filings  you  know  are  particles 
of  iron,  irregular  in  shape,  being  longer  in 
some  directions  than  in  others.  For  the  pres- 
ent experiment,  moreover,  instead  of  the  iroo 
filings,  very  small  scraps  of  thin  iron  wire 
might  be  employed.  I  place  a  sheet  of  paper 
over  the  magnet ;  it  is  all  the  better  if  the 
paper  be  stretched  on  a  wooden  frame,  as 
this  enables  us  to  keep  it  quite  level.  I  scat- 
ter the  filings,  or  the  scraps  of  wire,  from  a 
sieve  upon  the  paper,  and  tap  the  latter  gently, 
so  as  to  liberate  the  particles  for  a  moment 
from  its  friction.  The  magnet  acts  on  the  fil- 
ings through  the  paper,  and  see  how  it 
arranges  them  !  They  embrace  the  magnet 
in  u  series  of  beautiful  curves,  which  are 
technically  called  magnetic  curves,  or  lines 
of  magnetic  force.  t)oes  the  meaning  of 
these  lines  yet  flash  upon  you?  Set  your 
magnetic  needle  or  your  suspended  bit  of 
wire  at  any  point  of  one  of  the  curves,  and 
you  will  find  the  direction  of  the  needle  or  of 
the  wire  to  be  exactly  that  of  the  particle  cf 
iron,  or  of  the  magnetic  curve  at  the  point. 
Go  round  and  round  the  magnet  ;  the  direc- 
tion of  your  needle  always  coincides  with  the 
direction  of  the  curve  on  which  it  is  placed. 
These,  then,  are  the  lines  along  which  a  par- 
ticle of  south  magnetism,  if  you  could  detach 
it,  would  move  to  the  north  pole,  and  a  bit 
of  north  magnetism  to  the  south  pole  ;  they 
are  the  lines  along  which  the  decomposition 
of  the  neutral  fluFd  takes  place,  ancf  in  tho 
case  of  the  magnetic  needle,  one  of  its  poles 
being  urged  in  one  direction,  and  the  other 


LESSONS  IN  ELECTRICITY. 


341 


pole  in  the  opposite  direction,  the  needle  must 
necessarily  set  itself  as  a  tanffenttv  the  curve. 
I  \vill  not  seek  to  simplify  this  subject  fur- 
ther. If  there  be  anything  obscure  or  con- 
fused or  incomplete  in  my  statement,  you 
ought  now,  by  patient  thought,  to  be  able 
to  clear  away  the  obscurity,  to  reduce  the 
confusion  to  order,  and  to  supply  what  is 
needed  to  render  the  explanation  complete. 
Do  not  quit  the  subject  until  you  thoroughly 
understand  it  ;  and  if  you  are  able  to  look 
•with  your  mind's  eye  at  the  play  of  forces 
around  a  magnet,"  and  see  distinctly  the 
operation  of  those  forces  in  the  production  of 
the  magnetic  curves,  the  time  which  we  have 
spent  together  has  not  been  spent  in  vain. 

In  this  thorough  manner  we  must  master 
our  materials,  reason  upon  them,  and,  by  de- 
termined study,  attain  to  clearness  of  concep- 
tion. Facts  thus  dealt  with  exercise  an  ex- 
pansive force  upon  the  boundaries  of  thought; 
they  widen  the  mind  to  generalization.  We 
soon  recognize  a  brotherhood  between  the 
larger  phenomena  cf  Nature  and  the  minute 
effects  which  we  have  observed  in  our  private 
chambers.  Why,  we  inquire,  does  the  mag- 
netic needle  set  north  and  south  ?  Evidently 
it  is  compelled  to  do  sj  by  tho  earth  ;  the 
great  globe  which  we  inherit  is  itself  a  mag- 
net. Let  us  learn  a  lit  tie  more  about  it.  By 
means  of  a  bit  of  wax  or  otherwise,  attach 
your  silk  fibre  to  your  magnetic  needle  by  a 
single  point  at  its  middle,  the  needle  will  thus 
be  uuinterfered  with  by  the  paper  loop,  and 
will  enjoy  to  some  extent  a  power  of  dipping 
its  point  or  its  eye  below  the  horizon.  Lay 
your  magnet  on  a  table,  and  hold  the  needle 
over  the  equator  of  the  magnet.  The  needle 
sets  horizontal.  Move  it  toward  the  north 
end  of  the  magnet  ;  the  south  end  of  the 
needle  diijs,  the  dip  augmenting  as  you  ap- 
proach the  nortli  pole,  over  which  the  needle 
tf  free  to  move,  will  set  itself  exactly  vertical. 
Move  it  back  to  the  centre,  it  resumes  its 
konzontality  ;  pass  it  on  toward  the  south 
pole,  its  north  end  now  dips,  and  directly 
over  the  south  pole  the  needle  becomes  ver- 
tical, its  north  end  being  now  turned  down- 
ward. Thus  we  learn  that  on  the  one  side 
of  the  magnetic  equator  the  north  end  of  the 
needle  dips  ;  on  the  other  side  the  south  end 
dips,  the  dip  varying  from  nothing  to  ninety 
degrees.  If  we  go  to  the  equatorial  regions 
of  the  earth  with  a  suitably  suspended  needle, 
AVC  shall  find  there  the  position  of  the  needle 
horizontal.  If  we  sail  north,  one  end  of  the 
needle  dips;  if  we  sail  south,  the  opposite 
end  dips  ;  and  over  the  north  or  south  terres- 
trial magnetic  pole  the  needle  sets  vertical. 
The  south  magnetic  pole  has  not  yet  been 
found,,  but  Sir  James  Ross  discovered  the 
north  magnetic  pole  on  the  1st  of  June,  1881. 
In  this  manner  we  establish  a  complete  par- 
allelism between  the  action  of  the  earth  and, 
that  of  an  ordinary  magnet. 

The  terrestrial  magnetic  poles  do  not  coin- 
cide with  the  geographical  ones  ;  nor  does  tho 
earth's  magnetic  equator  quite  coincide  with 
the  geographical  emiator.  The  direction  of 
the  magnetic  neceiltt  in  London,  which  is 


called  the  magnetic  meridian,  incloses  an  an- 
gle of  24  degrees  with  the  1n:e  astronomical 
meridian,  this  angle  being  called  the  declina- 
tion of  the  needle  f<T  London.  The  north 
pole  of  the  needle  now  lirs  to  the  west  of  the 
true  meridian  ;  the  declination  is  westerly. 
In  the  year  1GGO,  however,  the  declination 
was  nothing,  while  before  that  lirnu  it  was 
casteily.  All  this  proves  that  the  earth's 
magnetic  constituents  are  gradually  changing 
iheir  distribution.  This  change  is  very 
slow:  it  is  technically  called  \l\esecularchange, 
and  the  observation  of  it  has  notj'et  extended 
over  a  sufficient  period  of  time  to  enable  us 
to  trucks,  even  approximately,  at  its  laws. 

Having  thus  discovered,  to  some  extent, 
the  secret  of  the  earth's  power,  we  can  turn 
it  to  account.  1  hold  in  my  hand  a  poker 
formed  of  good  soft  iron  ;  it  is  now  in  the  line 
of  dip,  a  tangent,  in  fact, to  the  earth's  line  of 
magnetic  force.  The  earth,  acting  as  a  mag- 
net, is  at  this  moment  constraining  the  two 
fluids  of  the  poker  to  separate,  making  th« 
lower  end  of  the  poker  a  north  pole,  and  th« 
upper  end  a  south  pole.  Matk  the  experi- 
ment :  I  hold  the  knob  uppermost,  and  it  at- 
tracts the  north  end  of  a  magnetic  needle.  1 
now  reverse  the  poker,  bringing  its  knob  un- 
dermost ;  the  knob  is  now  a  north  pole  and 
attracts  the  south  end  of  a  magnetic  needle. 
Get  such  a  poker  and  carefully  repeat  this 
experiment ;  satisfy  yourselves  that  the  fluids 
Biiift  their  position  according  to  the  manner 
in  which  the  poker  is  presented  to  the  caith. 
It  has  already  been  stated  that  the  softest 
iron  possesses  a  certain  amount  of  coerciyo 
force.  The  earth,  at  this  moment,  finds  in 
this  force  an  antagonist  which  opposes  the 
full  decomposition  of  the  neutral  fluid.  The 
component  fluids  may  be  figured  as  meeting 
an  amount  of  friction,  or  possessing  an 
amount  of  adhesion,  which  prevents  them 
from  gliding  over  the  atoms  of  the  poker. 
Can  we  assist  the  earth  in  this  case  ?  If  w« 
wish  to  remove  the  residue  of  a  powder  from 
the  interior  surface  of  a  glass  to  which  the 
powder  clings,  we  invert  the  glass,  tap  it, 
loosen  the  hold  of  the  powder,  and  thus  en- 
able the  force  of  gravity  to  pull  it  down.  80 
also  by  tapping  the  end  of  the  poker  we  loosen 
the  adhesion  of  the  fluid  to  the  atoms  and  en- 
able the  earth  to  pull  them  apart.  But  what 
is  the  consequence  ?  The  portion  of  fluid 
•which  has  been  thus  forcibly  dragged  over 
the  atoms  refuses  to  return  when  the  poker 
has  been  removed  from  the  line  of  dip  ;  tho 
iron,  as  you  see,  has  become  a  permanent 
magnet.  By  reversing  its  position  and  tap- 
ping it  again  we  reverse  its  magnetism.  A 
thoughtful  and  competent  teacher  will  well 
know  how  to  place  these  rcmarkabl'1  facts 
before  his  pupils  in  a  manner  which  will  ex- 
cite their  interest  ;  he  will  know,  and  if  not, 
will  try  to  learn,  how,  by  the  use  of  sensiblo 
images,  more  or  less  gross,  to  give  those  he 
teaches  definite  conceptions,  purifying  these 
conceptions  more  and  more  as  the  minds  of 
his  pupils  become  more  capable  of  abstraction. 
He  will  cause  his  logic  to  run  like  a  line  of 
light  through  these  images,  and  by  thus  act- 


LESSONS  IN  ELECTOICITT. 


Ing  he  will  cause  his  boys  to  march  at  his 
Bide  with  a  profit  and  a  joy,  which  the  mere 
exhibition  of  facts  without  principles,  or  the 
appeal  to  the  bodily  senses  and  the  power  of 
memory  alone,  could  never  inspire. 

As  an  expansion  of  the  note  at  page  339  the 
following  extract  may  find  a  place  here  : 

4 '  It  is  well  known  that  a  voltaic  current 
exerts  an  attractive  force  upon  a  second  cur- 
rent, flowing  in  the  same  direction  ;  and  that 
when  the  directions  are  opposed  to  each  other 
the  force  exerted  is  a  repulsive  one.  By  coil- 
ing wires  into  spirals.  Ampere  was  enabled 
to  make  them  produce  all  the  phenomena  of 
attraction  and  repulsion  exhibited  by  mag- 
nets, and  from  this  it  was  but  a  step  to  his 
celebrated  theory  of  molecular  currents.  He 
supposed  the  molecules  of  a  magnetic  body  to 
be  surrounded  by  such  currents,  which,  how- 
ever, in  the  natural  state  of  the  body  mutually 
neutralized  each  other,  on  account  of  their 
confused  grouping.  The  act  of  magnetization 
he  supposed  to  consist  in  setting  these  mole- 
cular currents  parallel  to  each  other  ;  and, 
starting  from  this  principle,  he  reduced  all  the 
phenomena  of  magnetism  to  the  mutual  action 
of  ecletric  currents. 

"If  we  reflect  upon  the  experiments  re- 
corded in  the  foregoing  pages  from  first  to 
last,  we  can  hardly  fail  to  be  convinced  that 
diamagnetic  bodies  operated  on  by  magnetic 
forces  possess  a  polarity  *  the  same  in  kind 
ns»  but  the  reverse  in  direction  of,  that  ac- 
quired by  magnetic  bodies.'  But,  if  this  be 
the  case,  how  are  we  to  conceive  the  physical 
mechanism  of  this  polarity  ?  According  to 
Coulcoin'j's  and  Poisson's  theory,  the  act  of 
magnetization  consists  in  the  decomposition 
of  a  neutral  magnetic  fluid  ;  the  north  pole  of 
a  magnet,  for  example,  possesses  an  attraction 
for  the  south  fluid  of  a  piece  of  soft  iron  sub- 


mitted to  its  influence,  draws  the  said  fluid 
toward  it,  and  with  it  the  material  particles 
with  which  the  fluid  is  associated.  To  ac- 
count for  diamagnetic  phenomena  this  theory 
seems  to  fail  altogether  ;  according  to  it,  in« 
deed,  the  oft  used  phrase,  'a  north  pole  ex- 
citing a  north  pole,  and  a  south  pole  a  south 
pole/  involves  a  contradiction.  For  if  the 
north  fluid  be  supposed  to  be  attracted  toward 
the  influencing  north  pole,  it  b absurd  to  sup- 
pose that  its  presence  there  could  produce  re- 
pulsion. The  theory  of  Ampere  is  equally  at 
a  loss  to  explain  diamagnetic  action  ;  for  if 
we  suppose  the  pai  tides  of  bismuth  sur 
rounded  by  molecular  cui  rents,  then,  accoid- 
ing  to  all  that  is  known  of  electro-dynamic 
laws,  these  currents  would  set  themselves 
parallel  to,  and  in  the  same  direction  as  those 
of  the  magnet,  and  hence  attraction,  and  not 
repulsion,  would  be  the  icsult.  The  fact, 
however,  of  this  not  being  the  case  proves 
that  these  molecular  curients  are  not  the 
mechanism  by  which  diamagnetic  induction 
is  effected.  The  consciousness  of  this,  I 
doubt  not,  drove  M.  Weber  to  the  assumption 
that  the  phenomena  of  diamagnetism  are  pro- 
duced by  molecular  currents,  not  directed,  but 
actually  excited  in  the  bismuth  by  the  magnet. 
Such  induced  currents  would,  according  to 
known  laws,  have  a  direct  ion  opposed  to  those 
of  the  inducing  magnet,  and  hence  would 
produce  the  phenomena  of  repulsion.  To 
carry  out  tb"  assumption  here  made.  M. 
Weber  is  obliged  to  suppose  that  the  mole- 
cules of  diamagnetic  bodies  are  surrounded 
by  channels,  in  which  the  induced  molecular 
currents,  once  excited,  continue  to  flow  with- 
out resistance." — Diam agnetism  and  Magne- 
crystalhc  Action,  pp.  13G,  137. 

THE  END. 


CONTENTS. 


.fttroductton 288 

JlUtoric  Notes 288 

T>\-3  Art  of  Experiment 289 

KUiCtJ-ic  Attractions 290 

I)is3>v-ry*  of  Conduction  and  Insulation 292 

T:i3  Electroscope &-3 

Electric  an,l  Non-Electrics 21)5 

Elactri-j  R-pulsions 208 

Ftmlirnsatal  Law  of  Electric  Action 297 

Double  or  "  Polar  "    Character  of  the  Electric 

Force   290 

What  is  Electricity? 301 

Electric  Induction 302 

The  Electropho*-us 3"7 

Action  of  Points  and  Flames 308 


The  Electrical  Machine 309 

The  Leyden  Jar 314 

Franklin's  Cascade  Battery 317 

Leyilen  Jars  of  the  Simplest  Form 318 

Itrnition  by  the  Electric  Spark 320 

Duration  of  the  Electric  Spark 3'J3 

Electric  Light  in  Vacuo 324 

Lichtenberg's  Figures 326 

Surface  Compared  with  Mass 3-26 

Physiological  Effects  of  the  Electrical  Discharge  328 

Atmospheric  Electricity 3"<i8 

The  Returning  Stroke 330 

The  Leyden  Battery 888 

Appendix:     An  Elementary  Lecture  on  Mag- 
netism    334 


SIX  LECTURES  ON  LIGHT. 


BY 

JOHN  TYNDALL. 


LECTURES  ON  LIGHT. 


CONTENTS: 

LECTURE  I. 
Introduction,     -  -,----       2 

LECTURE  II. 
Origin  of  Physical  Theories,  -  8 

LECTURE  III. 
Relation  of  Theories  to  Experience,  -  -      18 

LECTURE  IV. 
Chromatic  Phenomena  produced  by  Crystals,  -  -  27 

LECTURE  V. 
Range  of  Vision  and  Range  of  Radiation.     -  -  34 

LECTURE  VI. 
Spectrum  Analyses,  -  -  41 


SIX  LECTURES  ON  LIGHT. 


BY  Prof.  JOHN  TYNDALL,  F.R.S. 


PREFACE. 


MY  eminent  friend,  Prof.  Joseph  Henry, 
of  Washington,  did  me  the  honor  of  taking 
these  lectures  under  .his  personal  direction, 
and  of  arranging  the  times  and  places  at 
which  they  were  to  be  -delivered. 

Deeming  that  my  home-dud  .s  could  not, 
with  propriety,  be  suspended  for  a  longer 
period,  I  did  not,  at  the  outset,  expect  to  be 
able  to  prolong  my  visit  to  the  United 
States  beyond  the  end  of  1872. 

Thus  limited  as  to  time,  Prof.  Henry  began 
in  the  North,  and,  proceeding  southwards, 
arranged  for  the  successive  delivery  of  the 
lectures  in  Boston,  New  York,  Philadelphia, 
Baltimore,  and  Washington. 

By  this  arrangement,  which  circumstances 
at  the  time  rendered  unavoidable,  the  lec- 
tures in  New  York  were  rendered  coincident 
with  the  period  of  the  presidential  election. 
This  was  deemed  unsatisfactory,  and  when 
fhe  fact  was  represented  to  me  I  it  once  of- 
fered to  extend  the  time  of  my  visit  so  as  to 
'make  the  lectures  in  New  Ycrk  succeed 
those  in  Washington.  The  proposition  was 
cordially  accepted  by  my  friends. 

To  me  personally  this  modified  arrange- 
ment has  proved  in  the  highest  degree  satis- 
factory. It  gave  mt  a  much-needed  holiday 
at  Niagara  Falls  ;  it,  moreover,  rendered  the 
successive  stages  of  my  work  a  kind  oi grow'/i, 
which  reached  its  most  impressive  develop 
scent  in  New  York  and  Brooklyn. 


In  every  city  that  I  have  visited,  my  recep- 
tion has  been  that  of  a  friend  ;  and,  now 
that  my  visit  has  become  virtually  a  thing  of 
the  past,  I  can  look  back  upon  it  with  unqual- 
ified pleasure.  It  is  a  memory  without  a 
stain — an  experience  of  deep  and  genuine 
kindness  on  the  part  of  the  American  people 
never,  on  my  part,  to  be  forgotten. 

This  relates  to  what  may  be  called  the  pos- 
itive side  of  my  visit — to  the  circumstances 
attending  the  work  actually  done.  My  only 
drawback  relates  to  work  undone;  for  1  carry 
home  with  me  the  consciousness  of  having 
been  unable  to  respond  to  the  invitations  of 
the  great  cities  of  the  West ;  thus,  I  fear, 
causing,  in  many  cases,  disappointment. 
Would  that  this  could  have  been  averted  ! 
But  the  character  of  the  lectures,  and  the 
weight  of  instrumental  appliances  which  they 
involved,  entailed  loss  of  time  and  heavy 
labor.  The  need  of  rest  alone  would  be  a 
sufficient  admonition  to  me  to  pause  here  ; 
but,  besides  this,  each  successive  mail  from 
London  brings  me  intelligence  of  work  sus- 
pended and  duties  postponed  through  my 
absence.  These  are  the  considerations 
which  prevent  me  from  responding,  with  a 
warmth  commensurate  with  their  own,  to 
the  wishes  of  my  friends  in  the  West. 

On  quitting  England  I  had  no  intention 
of  publishing  these  lectures,  wnd,  except  a 
fragment  or  two,  not  a  line  of  them  was  writtes 


SIX  LECTURES  ON  LIGHT. 


when  1  reached  this  city.  They  have  been 
begun,  continued,  and  ended  in  New  York, 
and  bear  bniy  too  evident  marks  of  the  rapid- 
ity of  their  production.  I  thought  it,  how- 
ever, due.  bo;h  to  those  who  heard  them  with 
such  marked  attention,  and  to  those  who  wish- 
ed to  hear  them,  but  were  unable  to  do  so,  to 
leave  t.:em  behind  me  in  an  authentic  form. 
The  execution  of  this  work  has  cut  me  off 
from  many  social  pleasures  ;  it  has  also  pre- 
vented me  from  makingmyself  acquainted  with 
institutions  in  the  working  of  which  I  feel  a 
deep  interest.  But  human  power  is  finite, 
and  mine  has  been  expended  in  the  way  which 
I  deemed  most  agreeable,  not  to  my  more 
intimate  friends,  but  to  the  people  of  the 
United  States. 

In  the  opening  lecture  are  mentioned  the 
names  of  gentlemen  to  whom  I  am  under 
lasting  obligations  for  their  friendly  and  often 
laborious  aid.  The  list  might  readily  be  ex- 
tended, for  in  every  city  I  have  visited  willing 


helpers  were  at  hand.  I  must  not,  however, 
omit  the  name  of  Mr.  Rhets,  Professor 
Plenry's  private  secretary,  who,  not  only  in 
Washington,  but  in  Boston,  gave  me  most 
important  assistance.  To  the  trustees  of  the 
Cooper  Institute  my  acknowledgments  are 
due  ;  also  to  the  directors  of  the  Mercantile 
Library  at  Brooklyn.  I  would  add  to  these  a 
brief  but  grateful  reference  to  my  high-miuded 
friend  and  kinsman,  General  Hector  Tyn- 
dale,  for  his  long-continued  care  of  me,  and  for 
the  thoughtful  tenderness  by  which  he  and  his 
family  softened,  both  to  me  and  to  the  parents 
of  the>vuth,  the  pain  occasioned  by  the  death 
of  my  junior  assistant  in  Philadelphia. 

Finally,  I  have  to  mention  with  warm  com- 
mendation the  integrity,  ability,  and  devo- 
tion, with  which,  from  first  to  last,  I  have 
been  aided  by  my  principal  assistant,  Mr. 
John  Cottrell. 

NEW  YORK,  February,  1873. 


LECTURE  I. 

INTRODUCTORY  :  Uses  of  Experiment :  Early  Scien- 
tific Notions:  Sciences  of  Observation:  Knowl- 
edge of  the  Ancients  Regarding  Light :  Nature 
judged  from  Theory  defective:  Detects  of  the 
Eye:  Our  Instruments:  Rectilineal  Propagation 
of  Light :  Law  of  Incidence  and  Reflection : 
Sterility  of  the  Middle  Ages:  Refraction:  Dis- 
covery of  Snell :  Descartes  and  the  Rainbow  : 
Newtonrs  Experiments  on  the  Composition  of 
Solar  Light :  His  Mistake  as  regards  Achroma- 
tism :  Synthesis  ot  White  Li<ht :  Yellow  and 
lUut:  Lights  proved  to  produce  White  by  their 
Mixture:  Colors  of  Natural  Bodies:  Absorption: 
Mixture  of  Pigments  contrasted  with  Mixture  of 
LLhts. 

SOME  twelve  years  ago  I  published,  in 
England,  a  little  book  entitled  the  "  Glaciers 
of  the  Alps,"  and,  a  couple  of  years  subse- 
quently, a  second  volume,  entitled  "  Heat  as 
a  Mode  of  Motion."  These  volumes  were 
followed  by  others,  written  with  equal  plain- 
ness, and  with  a  similar  aim,  that  aim  being 
to  develop  and  deepen  sympathy  between 
science  and  the  world  outside  of  science.  I 
agreed  with  thoughtful  men*  who  deemed 
it  good  for  neither  world  to  be  isolated  from 
the  other,  or  unsympathetic  towards  the 
other,  and,  to  lessen  this  isolation,  at  least  in 
one  department  of  science,  I  swerved  aside 
from  those  original  researches  which  had  pre- 
viously been  the  pursuit  and  pleasure  of  my 
life. 

These  books  were,  for  the  most  part,  re- 
published  by  the  Messrs.  Appleton,  under 


*Among  whom  may  be  mentioned,  specially,  the 
l»:e  Sir  Edmund  Head,  Bart. 


the  auspices  of  a  man  who  is  untiring  in  his 
efforts  to  diffuse  sound  scientific  knowledge 
among  the  people  of  this  country;  whose 
energy,  ability,  and  single-mindedness,  in  the 
prosecution  of  an  arduous  task,  have  won  for 
him  the  sympathy  and  support  of  many  of  us 
in  "the  old  country."  1  allude  to  Professor 
Youmans,  of  this  city.  Quite  as  rapidly  as 
in  England,  the  aim  of  these  works  was  un- 
derstood and  appreciated  in  the  United 
States,  and  they  brought  me  from  this  side 
of  the  Atlantic  innumerable  evidences  of 
good-will.  Year  after  year,  invitations 
reached  me  *  to  visit  America,  and  last  year 
I  was  honored  with  a  request  so  cordial,  and 
signed  by  five-and-twenty  names  so  distin- 
guished in  science,  in  literature,  and  in  ad- 
ministrative position,  that  I  at  once  resolved 
to  respond  to  it  by  braving,  not  only  the  dis- 
quieting oscillations  of  the  Atlantic,  but  the 
far  more  disquieting  ordeal  of  appearing  in 
person  before  the  people  of  the  United 
States.  , 

This  request,  conveyed  to  me  by  my  ac- 
complished friend,  Professor  Lesley,  of  Phil- 
adelphia, and  preceded  by  a  letter  of  the 
same  purport  from  your  scientific  Nestor, 
Professor  Joseph  Henry,  of  Washington,  de- 
sired that  I  would  lecture  in  some  of  the 
principal  cities  of  the  Union.  This  I  agreed 
to  do,  though  much  in  the  dark  as  to  what 
foim  such  lectures  ought  to  to  take.  In 

*  One  of  the  earliest  came  from  Mr.  John  Amory 
Lowell,  of  Boston. 


SIX  LECTURES  ON  LIGHT 


answer  to  my  inquiries,  however,  I  was  given 
to  understand  (by  Professor  Youmans  princi- 
pally) that  a  course  of  experimental  lectures 
would  materially  promote  scientific  education 
in  this  country,  and  I  at  once  resolved  to 
meet  this  desire,  as  far  as  my  time  allowed. 

Experiments  have  two  uses — a  use  in  dis- 
covery, and  a  use  in  tuition.  They  are  the 
investigator's  language  addressed  to  Nature, 
to  which  she  sends  intelligible  replies.  These 
replies,  however,  are,  for  the  most  part,  at 
first  too  feeble  for  the  public  ear  ;  for  the  in- 
vestigator cares  little  for  the  loudness  of  Na- 
ture's voice  if  he  can  only  unravel  its  meaning. 
But  after  the  discoverer  comes  the  teacher, 
whose  function  it  is  so  to  exalt  and  modify  the 
resu  ts  of  the  discoverer  as  to  render  them  fit 
for  public  presentation.  This  secondary 
function  I  shall  endeavor,  in  the  present  in- 
stance, to  fulfil. 

I  propose  to  take  a  single  department  of 
natural  philosophy,  and  illustrate,  by  means 
of  it,  the  growth  of  scientific  knowledge  under 
the  guidance  of  experiment.  I  wish,  in  this 
ii  st  lecture,  to  make  you  acquainted  with  cer- 


single  increment  is  made  good  by  the  indefi- 
nite number  of  su-ch  increments,  summed  up 
in  what  may  be  regarded  as  practically  infinite 
time. 

We  will  not  now  go  back  to  man's  first 
intellectual  gropings  ;  much  less  shall  we 
enter  upon  the  thorny  discussion  as  to  how 
the  groping  man  arose.  We  will  take  him  at 
a  certain  stage  of  his  development,  when,  fay 
evolution  or  sudden  endowment,  he  became 
possessed  of  the  apparatus  of  thought  and 
the  power  of  using  it.  For  a  time — and  that 
historically  a  long  one — he  was  limited  to 
mere  observation,  accepting  what  Nature  of- 
fered, and  confining  intellectual  action  to  it. 
The  apparent  motions  of  sun  and  stars  first 
drew  towards  them  the  questionings  of  the  in- 
tellect, and  accordingly  astronomy  was  the 
first  science  developed.  Slowly,  and  with  difE.- 
culty,  the  notion  of  natural  forces  took  root 
in  the  mind,  the  seedling  of  this  notion  being 
the  actual  observation  of  electric  and  mag- 
netic attractions.  Slowly,  and  with  difficulty, 
the  science  of  mechanics  had  to  grow  out  of 
this  notion  ;  and  slowly  at  last  came  the  full 


tain  elementary  phenomena  ;  then  to  point  application  of  mechanical  principles  to  the 
out  to  you  how  those  theoretic  principles  by  motions  of  the  heavenly  bodies.  We  trace 
which  phenomena  are  explained,  take  rooi,  I  the  progress  of  astronomy  through  Hip- 
and  flourish  in  the  human  mind,  and  after-  J  parchus  and  Ptolemy;  and,  after  a  long  halt, 
wards  to  apply  these  principles  to  the  whole  through  Copernicus,  Galileo,  Tycho  Bratue, 


and  Kepler;  w  ile,  from  the  high  table-land 
of   thought   raised   by   these   men,    Newton-. 


tx  dy  of  knowledge  covered  by  the  lectures. 

The  science  of  optics  lends  itself  to  this  mode 

of   reatment,  and  on  it,  therefore,  I  propose  Lshoots  upward  like  a  peak,  overlooking  all 

to  draw  for  the  materials  of  the  present  course.  I  others  from  his  dominant  elevation. 

It  will  be  best  to  begin  with  the  few  simple  I      But  other  objects  than  the  motions  of  the 

facts  regarding  light  which  were  known  to  the    stars  attracted  the  attention  of  the   ancient 


ancients,  and  to  pass  from  them  in  historic 
gradation  to  the  more  abstruse  discoveries  of 
modern  times. 

All  our  notions  of  Nature,  however  exalted 


world.  Light  was  a  familiar  phenomenon, 
and  from  the  earliest  times  we  find  men's 
rr.inds  busy  with  the  attempt  to  render  some 
account  of  it.  But,  without  experiment, 


or  however  grotesque,  have  some  foundation  ;  which  belongs  to  a  later  stage  of  scientific 

development,  little  progress  could  be  madein 
this    subject.       The   ancients,    accordingly, 


in  experience.  The  notion  of  personal  voli- 
tion in  Nature  had  this  basis.  In  the  fury 
and  the  serenity  of  natural  phenomena  the 
savage  saw  the  transcript  of  his  own  varying 
moods,  and  he  accordingly  ascribed  these 
phenomena  to  beings  of  like  passions  with 
himself,  but  vastly  transcending  him  in  power. 
Thus  the  notion  of  causality — the  assumption 
that  natural  things  did  not  com  2  of  themselves, 


were  far  less  successful  in  dealing  with  light 
than  in  dealing  with  solar  and  stellar  mo- 
tions. Still,  they  did  make  some  progress. 
They  satisfied  themselves  that  light  moved 
in  straight  lines;  they  knew,  also,  that  these 
lines  or  rays  of  light  were  reflected  from  pol- 
ished surfaces,  and  that  the  angle  of  inei- 

but  had  unseen  antecedents — lay  at  the  root  \  dence  was  equal  to  the  angle  of  reflection, 
of  even  the  savage's  interpretation  of  Nature.  !  These  two  results  of  ancient  scientific  curios- 
Out  of  this  bias  of  the  human  mind  to  seek  j  ity  constitute  the  starting-point  of  our  pres- 
for  the  antecedents  of  phenomena  all  science  '  ent  course  of  lectures. 

But,  in  the  first  place,  it  may  be  useful  to 
say  a  few  words  regarding  the  source  of  light' 
to  be  employed  in  our  experiments.  The, 


has  sprung. 

The  development  of  man,  indeed,  is  ulti- 
mately due  to  his  interaction  with  Nature. 
Natural  phenomena  arrest  his  attention  and 
excite  his  questionings,  the  intellectual  activity 


rusting  of  iron  is,  to  all  intents  and  purposes, 
the  slow  burning  of  iron.     It   develops  heat, 

thus  provoked  reacting  on  the  intellect  itself,  j  and,  if  the  heat  be  preserved,  a  high  temper- 
and  adding  to  its  strength.  The  quantity  of  j  ature  may  be  thus  attained.  The  destruc- 
power  added  by  any  single  effort  of  the  in-  tion  of  the  first  Atlantic  cable  was  probably 
tellect  may  be  indefinitely  small  ;  but  the  in-  due  to  heat  developed  in  this  way.  Other 
tegration  of  innumerable  increments  of  this  J  metals  are  still  more  combustible  than  iron, 
kind  has  raised  intellectual  power  from  its  j  You  may  light  strips  of  zinc  in  a  candle- 
rudiments  to  the  magnitude  it  possesses  to-  flame,  and  cause  them  to  burn  almost  like 
day.  In  fact,  the  indefinite  smallness  of  the  strips  of  paper.  But,  besides  combustion  in 


SIX  LECTURES  ON  LIGHT. 


the  air,  we  may  also  have  combustion  it*  a 
liquid.  Wat«r,  for  example,  contains  a  store 
of  oxygen  which  may  unite  with  and  consume 
a  metal  immersed  in  it.  It  is  from  this  kind 
of  combustion  that  we  are  to  derive  the  heat 
and  light  employed  in  the  present  course. 

Their  generation  merits  a  moment's  atten- 
tion. Before  you  is  an  instrument — a  small 
voltaic  battery — in  which  zinc  is  immersed  in 
!  a  suitable  liquid.  Matters  are  so  arranged 
that  an  attraction  is  set  up  between  the  metal 
and  the  oxygen,  actual  union,  however,  being 
in-  the  first  instance  avoided.  Uniting  the 
two  ends  of  the  battery  by  a  thick  wire,  the 
attraction  is  satisfied,  the  oxygen  unites  with 
the  metal,  the  zinc  is  consumed,  and  heat,  as 
usual,  is  the  result  of  the  combustion.  A 
power,  which,  for  want  of  a  better  name,  we 
call  an  electric  current,  passes  at  the  same 
time  through  the  wire. 

Cutting  the  thick  wire  in  two,  I  unite  the 
severed  ends  by  a  thin  one.  It  glows  with  a 
white  heat.  Whence  comes  that  heat  ?  The 
question  is  well  worthy  of  an  answer.  Sup- 
pose in  the  first  instance,  when  the  thick  wire 


to  form  the  image.  It  H  not  sharp,  but  sur, 
rounded  by  a  halo  whic'i  nearly  obliterates  it. 
This  arises  from  an  imperfection  of  the  lens, 
called  its  spJierical  aberration,  due  to  the  fact 
that  the  circumferential  and  central  rays  have 
not  the  same  focus.  The  human  eye  labors 
under  a  similar  defect,  and,  when  you  looked 
at  the  naked  light  from  fifty  cells,  the  blur  of 
light  upon  the  retfna  was  sufficient  to  destroy 
the  definition  of  th<;  retinal  image  of  the  car- 
bons. A  long  list  of  indictments  might  in- 
deed be  brought  against  the  eye — its  opacity, 
its  want  of  symmetry,  frs  lack  of  achroma- 
tism, its  absolute  blindness,  in  pa;t.  All 
these  taken  together  caused  an  eminent  Ger- 
man philosopher  to  say  that,  if  any  optician 
sent  him  an  instrument  so  full  of  defects,  he 
would  send  it  back  to  him  with  the  severesf 
censure.  But  the  eye  is  not  to  be  judged 
from  the  standpoint  of  theory.  As  a  practi' 
cal  instrument,  and  taking  the  adjustments  by 
which  its  defects  are  neutralized  into  account, 
it  must  ever  remain  a  marvel  to  the  reflecting 
mind 

Tae  ancients,  as  I  have  said,  were  aware  o/ 


was  employed,  that  we  had  permitted  the  ac-  |  the  rectilineal   propagation  of   light.     They 


lion  to  continue  until  100  grains  of  zinc  were 
consumed,  the  amount  of  heat  generated  in 
the  battery  would  be  capable  of  accurrte  nu- 
merical expression.  Let  the  action  now  con- 
tinue, with  this  thin  wire  glowing,  until  100 


knew  that  an  opaque  body,  placed  between 
the  eye  and  a  point  of  light,  intecepted  the 
light  of  the  point.  Possibly  the  terms  "  ray  " 
and  "beam"  may  have  been  suggested  by 
those  straight  spokes  of  light  which,  in  c^r 


grains  of  zinc  are  consumed.     Will  the  amount  j  tain  states  of  the  atmosphere,  dart  from  thi 


t-f  heat  generated  in  the  battery  be  the  same 
as  before  ?  No,  it  will  b>i  less  by  the  precise 
amount  generated  in  the  thin  wire  outside  the 
battery.  In  fact,  by  adding  the  internal  heat 
to  the  external,  we  obtain  for  the  combustion 
of  100  grains  of  zinc  a  total  which  never  va- 
ries. By  this  arrangement,  then,  we  are  able 
to  burn  our  zinc  at  one  place,  and  to  exhibit 
the  heat  and  light  of  its  combustion  at  a  dis- 
tant place.  In  New  York,  for  example,  we 
have  our  grate  and  fuel  ;  but  the  heat  and 
light  of  our  fire  may  be  made  to  appear  at 
San  Francisco. 

I  now  remove  the  thin  wire  and  attach  to 
the  severed  ends  of  the  thick  one  two  thin 
rods  of  coke.  On  bringing  the  rods  together 
we  obtain  a  small  star  of  light.  Now,  the 
light  to  be  employed  in  our  lectures  is  a  sim- 
ple exaggeration  of  this  star.  Instead  of 
being  produced  by  ten  cells,  it  is  produced  by 
fifty.  Placed  in  a  suitable  camera,  provided 
with  a  suitable  lens,  this  light  will  give  us  all 
the  beams  necessary  for  our  experiments. 

And  here,  in  passing,  let  me  refer  to  the 


sun  at  his  rising  and  his  setting.  The  recti-V 
lineal  propagation  of  light  may  be  illustrated 
at  home  in  th:s  way:  Make  a  small  hole  in  a 
closed  window-shutter,  before  which  stands  a 
house  or  a  tree,  and  place  within  the  dark- 
ened room  a  white  screen  at  some  distance 
from  the  orifice.  Every  straight  ray  proceed- 
ing from  the  house  or  tree  stamps  its  color 
upon  the  screen,  and  the  sum  of  all  the  rays 
forms  an  image  of  the  object.  But,  as  the 
rays  cross  each  other  at  the  orifice,  the  image 
is  inverted.  Here  we  may  illustrate  the  sub- 
ject thus:  In  front  of  our  camera  is  a  large 
opening,  closed  at  present  by  a  sheet  of  tin- 
foil. Pricking  by  means  of  a  common  sew- 
ing-needle a  small  aperture  in  the  tin-foil,  an 
inverted  image  of  the  carbon-points  starts 
forth  upon  the  screen.  A  dozen  apertures 
will  give  a  dozen  images,  a  hundred  a  hun- 
dred, a  thousand  a  thousand.  But.  as  the 
apertures  come  closer  to  each  other,  that  is  to 
say,  as  the  tin-foil  between  the  apertures  van- 
ishes, the  images  overlap  more  and  more. 
Removing  the  tin-foil  altogether,  the  screen 


common  delusion  that  the  works  of  Nature,  I  becomes  uniformly   illuminated      Hence  the 
the  human  eye  included,  are  theoretically  per-  |  light  upon  the  screen  may  be  regarded  as  the 

overlapping  of  innumerable  images  of  the 
carbon-points.  In  like  manner  the  light 
upon  every  white  wall  on  a  cloudless  day 
may  be  regarded  as  produced  by  the  super- 
position of  innumerable  images  of  the  sun. 

The   law  that  the  angle   of    incidence   is 
equal  to  the  angle  of  reflection  is  illustrated 


The  degree  of  perfection  of  any  organ 
is  determined  by  what  it  has  to  do.  Looking 
at  the  dazzling  light  from  our  large  battery, 
you  see  a  globe  of  light,  but  entirely  fail  to 
see  the  shape  of  the  coke-points  whence  the 
light  issues.  The  cause  may  be  thus  made 
clear  :  On  the  screen  before  you  is  projected 


an.  image  of  the  carbon-points,  the  whole  of  j  in  this  simple  way:  A  straight  lath  is  placed 
the  lens  in  front  of  the  camera  being  employed  I  as  an  index  perpendicular  to  a  small  looking- 


SIX  LECTURES  ON  LIGHT. 


glass  capable  of  rotation.  A  beam  of  light 
is  received  upon  the  glass  and  reflected  back 
upon  the  line  of  its  incidence.  Though  the 
incident  and  the  reflected  beams  pass  in 
opposite  directions,  they  do  not  jostle  or  dis- 
place each  other.  The  index  being  turned, 
the  mirror  turns  along  with  it,  and  at  each 
side  of  the  index  the  incident  and  the 
reflected  beams  are  seen  tracking  themselves 
through  the  dust  of  the  room.  The  mere 
inspection  of  the  two  angles  enclosed  be- 
tween the  index  and  the  two  beams  suffices 
to  show  their  equality.  The  same  simple 
apparatus  enables  us  to  illustrate  a  law  of 
great  practical  importance,  name  y,  that, 
when  a  mirror  rotates,  the  angular  velocity 
of  a  beam  reflected  from  it  is  twic^  that  of 
the  reflecting  mirror.  One  experiment  will 
make  this  pla  n  to  you.  The  mirror  is 
now  vertical,  and  both  the  incident  and  the 
reflected  beams  are  horizontal.  Turning  the 
mirror  through  an  angle  of  45°  the  reflected 
beam  is  vertical ;  that  is  to  say,  it  has  moved 
9Or ,  or  through  twice  the  angle  of  the  mirror. 

One  of  the  problems  of  science,  on  which 
scientific  progress  mainly  depends,  is  to  help 
the  senses  of  man  by  carrying  them  into  re- 
gions which  could  never  be  attained  without 
such  help.  Thus  we  arm  the  eye  with  the 
telescope  when  we  want  to  sound  the  depths 
of  space,  and  with  the  miscroscope  when  we 
want  to  explore  motion  and  structure  in  their 
infinitesimal  dimensions.  Now,  this  law  of 
angular  reflection,  coupled  with  the  fact  that 
a  beam  of  light  possesses  no  weight,  gives  us 
the  means  of  magnifying  small  motions  to  an 
extraordinary  degree.  Thus,  by  attaching 
mirrors  to  his  suspended  magnets,  and  by 
wa  ching  the  images  of  scales  reflected  from 
the  mirrors,  the  celebrated  Gauss  was  able  to 
detect  the  slightest  thrill  or  variation  on  the 
part  of  the  earth's  magnetic  force.  The  mi- 
nute elongation  of  a  bar  of  metal  by  the  mere 
warmth  of  the  hand  may  be  so  magnified  by 
this  method  as  to  cause  the  index-beam  to 
move  from  the  ceiling  to  the  floor  of  this 
room.  The  elongation  of  a  bar  of  iron  when 
it  is  magnetized  may  be  thus  demonstrated. 
By  a  similar  arrangement  the  feeble  attrac- 
tions and  repulsions  of  the  diamagnetic  force 
have  been  made  manifest;  while  in  Sir  William 
Thompson's  reflecting  galvanometer  the  prin- 
ciple receives  one  of  its  latest  applications. 

For  more  than  1,000  years  no  step  was 
taken  in  optics  beyond  this  law  of  reflection. 
The  men  of  the  Middle  Ages,  in  fact,  endeav- 
ored on  the  o  ,e  hand  to  develop  the  laws  of 
the  universe  out  of  their  own  consciousness, 
while  many  of  them  were  so  occupied  with 
the  concerns  of  a  future  world  that  they 
looked  with  a  lofty  scorn  on  all  things  pertain- 
ing to  this  one.  Notwithstanding  its  demon- 
strated failure  during  1,500  years  of  trial, 
there  are  still  men  among  us  who  think  the 
riddle  of  the  universe  is  to  be  solved  by  this 
appeal  to  consciousness.  And,  like  most 
people  who  support  a  delusion,  they  maintain 


theirs  warmly,  and  show  scant  respect  for 
those  who  dissent  from  their  views.*  As  re- 
gards the  refraction  of  light,  the  course  of 
real  inquiry  was  resumed  in  noo  by  an  Ara- 
bian philosopher  named  Alhazen.  Then  it 
was  taken  up  in  succession  by  Roger  Bacon, 
Vitellio,  and  Kepler.  One  of  the  most  im- 
portant occupations  of  science  is  the  deter- 
mination, by  precise  measurements,  of  the 
I  quantitative  relations  of  phenomena.  The 
value  of  such  measurements  depends  upon  the 
skill  and  conscientiousness  of  the  man  who 
makes  them.  Vitellio  appears  to  have  been 
both  skilful  and  conscientious,  while  Kepler's 
nabit  was  to  rummage  through  the  ob.-erva- 
tions  of  his  predecessors,  look  at  them  in  ail 
lights,  and  thus  distill  from  them  the  princi- 
ples which  united  them.  He  had  done  this 
with  the  astronomical  measurements  of 
Tycho  Brahe,  and  had  extracted  from  them 
the  celebrated  "  laws  of  Kepler."  He  did  k 
also  with  the  measurements  of  Vitellio.  ^3ut 
in  the  case  of  refraction  he  was  not  success- 
ful. The  principle,  though  a  simple  one,  es- 
caped him.  It  was  firs  discovered  by  \Ville- 
brod  Snell,  about  the  year  1621. 

Less  with  the  view  of  dwelling1  upon  the 
phenomenon  itself  than  of  introducing  it  to 
you  in  a  form  which  will  render  intelligible 
the  play  of  theoretic  thought  in  Newton's 
mind,  I  will  show  you  the  fact  of  refraction. 
The  dust  of  the  air  and  the  turbidity  of  a  liquid 
may  here  be  turned  to  account.  A  shallow 
circula-  vessel  with  a  glass  face,  half  rilled 
with  water,  rendered  barely  turbid  by  the 
precipitation  of  a  little  mastic,  is  placed  upon 
its  edge  with  its  glass  face  vertical.  Through 
a  slit  in  the  hoop  surrounding  the  vessel  a 
beam  of  light  is  admitted.  It  impinges  upon 
the  water,  enters  it,  and  tracks  itself  through 
the  liquid  in  a  sharp,  bright  band.  Meanwhile 
the  beam  passes  unseen  through  the  air  above 
the  water,  for  the  air  is  not  competent  to 
scatter  the  light.  A  puff  of  tobacco  smoke 
into  this  space  at  once  reveals  the  track  of  the 
incident-beam.  If  the  incidence  be  vertical, 
the  beam  is  unrefracted.  If  oblique,  its  re- 
fraction at  the  common  surface  of. air  and 
water  is  rendered  clearly  visible.  It  is  also 
seen  that  reflection  accompanies  refraction, 
the  beam  dividing  itself  at  the  point  of  inci- 
dence into  a  refracted  and  a  reflected  portion. 
The  law  by  which  Snell  connected  together 
all  the  measurements  executed  up  to  his  time, 
is  this  :  Let  A  B  C  D  represent  the  outline 
of  our  circular  vessel  (Fig.  i),  A  C  being  the 
water-line.  When  the  beam  is  incident  along 
B  E,  which  is  perpendicular  to  A  C,  there  is 
no  refraction.  When  it  is  incident  along  m 
E,  there  is  refraction  :  it  is  bent  at  E  and 
strikes  the  circle  at  n.  When  it  is  incident 


*  Schelling  thus  expresses  his  contempt  for  experi- 
mental knowledge  :  "  Newton's  Optics  is  the  greatest 
illustration  of  a  whole  structure  of  fallacies,  which  in 
all  its  parts  is  founded  on  observation  and  experi- 
ment." There  are  some  small  imitators  of  Schelling 
still  in  Germany. 


SIX  LECTURES  ON  LIGHT. 


along  mr  E,  there  is  also  refraction  at  E,  the 
beam  striking  the  point  nf .  From  the  ends 
of  the  incident  beams,  let  the  perpendiculars 
111  o,  m'  o/  ba  drawn  upon  1)  D,  and  from  the 
ends  of  the  refr-cted  beams  let  the  perpen- 
diculars p  n,p'  nf  be  also  drawn.  Measure 
the  lengths  of  o  in  and  of  p  n.  and  divide  the 


one  by  the  ether.  You  obtain  a  certain  quo- 
tient. In  like  manner  divide  m'  of  by  the 
corresponding  perpendicular  pr  ;/;  you  ob- 
tain in  each  case  the  same  quotient.  Snell,  in 
fact,  found  this  quotient  to  be  a  constant 
quantity  for  each  particular  substance,  though 
ic  varied  in  amount  from  substance  to  sub- 
st.mce  He  called  the  quotient  the  index  of 
refraction. 

This  law  is  one  of  the  corner-stones  of 
optical  science,  and  its  applications  to-day 
are  million-fold.  Immediately  after  its  dis- 
covery, Descartes  applied  it  to  the  explana- 
tion of  the  rainbow.  The  bow  :s  seen  when 
tbe  back  is  turned  to  the  sun.  Draw  a 
straight  line  through  the  spectator's  eye  and 
the  sun,  the  bow  is  always  seen  at  the  same 
angular  distance  from  this  line.  This  was 
the  great  difficulty.  Why  should  the  bow  be 
always  and  at  all  its  parts,  forty-one  degrees 
from  this  line  ?  Taking  a  pen  and  calculat- 


certain  that  he  did  net  enunciate  the  true 
law.  This  was  reserved  for  Newton,  who 
went  to  work  in  this  way:  Through  the  closed 
window-shutter  of  a  room  he  pieiced  an  ori- 
fice, and  allowed  a  thin  sunbeam  to  pass 
through  it.  The  beam  stamped  a  round 
image  of  the  sun  on  the  opposite  white  wall 
of  the  room.  In  the  path  of  this  beam  New- 
ton placed  a  prism,  expecting  to  see  the  beam 
refracted,  but  also  expecting  to  see  the  image 
of  the  sun,  af'cr  refi action,  round;  to  his 
astonishment,  it  was  drawn  out  to  an  image 
whose  length  was  five  times  its  breadth;  and 
this  image  was  divided  into  bands  of  differ- 
ent colors.  Newton  saw  immediately  that 
solar  light  was  composite,  not  simple.  His 
image  revealed  to  him  the  fact  that  some  con- 
stituents of  the  solar  light  were  more  deflect- 
ed by  the  prism  than  others,  and  he  conclud- 
ed, therefore,  that  white  solar  light  was  a 
mixture  of  lights  of  different  colors  and  of 
different  degrees  of  rcfrangibility. 

Let  us  reproduce  this  celebrated  experi- 
ment. On  tne  screen  is  now  stamped  a  lu- 
minous disk,  which  may  stand  for  Newton's 
image  of  the  sun.  Causing  the  beam  wl  ich 
produces  the  disk  to  pass  through  a  pri-rn, 
we  obtain  Newton's  elongated  colored  image, 
which  he  called  a  spectrum.  Newton  divided 
the  spectrum  into  seven  parts — red,  orange, 
yellow,  green,  blue,  indigo,  violet — which 
are  commonly  called  the  seven  primary  or 
prismatic  colors.  This  drawing  out  of  the 
white  light  into  its  constituent  colors  is  called 
dispersion. 

This  was  the  first  analysis  of  solar  light  bv 
Newton  ;  but  the  scientific  mind  is  fond  of 
verification,  and  never  neglects  it  where  it  is 
possible.  It  is  this  stern  conscientiousness  in 
testing  its  conclusions  that  gives  adamantine 
trength  to  science,  and  renders  all  assaults 
on  it  unavailing.  Newton  completed  his 
proof  by  synthesis  in  this  way  :  The  spec- 
rum  now  before  you  is  produced  by  a  glass 
prism.  Causing  the  decomposed  beam  to 


ing  the  track  of  every  ray  through  a  rain-    pass  through  a  second  simitar  prism,  but  so 
drop,  Descartes  found  that,  at  one  particular    placed  that  the  colors  are  refracted  back  and 


angle,  the  rays  emerged  from  the  drop  almost 
parallel  to  each  other;  being  t  :us  enabled  to 
preserve  their  intensity  through  long  atmos- 
pheric distances;  at  all  other  angles  the  rays 
quitted  the  drop  divergent,  and  through  this 
divergence  became  so  enfeebled  as  to  be 
practically  ;ost  to  the  eye.  The  particular 
angle  here  referred  to  was  the  ioregoing 
angle  of  forty-one  degrees,  which  observa- 
tion had  proved  to  be  invariably  that  of  the 


rainbow. 
But   in 


the   rainbow  a  new  phenomenon 


was  introduced — the  phenomenon  of  color. 
And  here  we  arrive  at  one  of  those  points  in 
the  history  of  science,  when  men's  labors  so 
intermingle,  that  it  is  difficult  to  assign  to 
each  worker  his  precise  meed  cf  honor.  Des- 
cartes was  at  the  threshold  of  the  discovery 
of  the  composition  of  solar  light.  But  he 
failed  to  attain  perfect  clearness,  and  it  is 


reblended,  the  perfectly  white  image  of  the 
slit   is  restored.     Here,  then,  refraction  and 
dispersion  are  simultaneously  abolished.     Are 
they  always  so  ?     Can  we  have  the  one  with- 
out the  other?     It  was  Newton's  conclusion 
that  we  could  not.     Here  he  erred,  and  his 
error,  which  he  maintained  to  the  end  of  his 
life,  retarded  the  progress  of  optical  discovery. 
Dolland  subsequently   proved  that,  by  com 
bining  two  different  kinds  of  glass,  the  color 
could  be  extinguished,  still  leaving  a  rcsidui 
of  refraction,  and  he  employed  this  residue 
in   the   construction  of   achromatic   lenses  - 
lenses  which  yield  no  color — which  Newion 
thought  an  impossibility.     By  setting  a  water 
prism — water   contained    in  a  wedge-shaped 
vessel   with  glass   sides — in  opposition    to  a 
prism  of  glass,  this  point  can   be  illustrated 
before  you.     We  have  first  the  position  of  the 
unrefracted  beam  marked  upon  the  screen  ; 


SIX  LECTURES  ON  LIGHT, 


then  we  produce  the  water-spectrum  ;  finally, 
by  introducing  a  flint  glass  prism,  we  refract 
the  beam  back,  until  the  color  cisappears. 
The  image  of  the  slit  is  now  white  ;  but  you 
see  that,  though  the  dispersion  is  abolished, 
the  refraction  is  not. 

This  is  the  place  to  illustrate  another  point 
bearing  upon  the  instrumental  means  em- 
ployed in  these  lectures.  Note  the  position 
ot  the  water-spectrum  upon  the  screen.  Alter- 
ing, in  no  particular,  the  wedge-shaped  ves- 
sel, but  simply  substituting  for  the  water  the 
transparent  bisulphide  of  carbon,  you  notice 
how  much  higher  the  beam  is  thrown,  and 
how  much  richer  is  the  display  of  color. 
This  will  explain  to  you  the  use  of  this  sub- 
stance in  our  subsequent  experiments. 

The  synthesis  of  white  light  may  be 
effected  in  three  ways,  which  are  now  worthy 
of  special  attention:  Here,  in  the  first  in- 
stance, we  have  a  rich  spectrum  produced  by 
a  prism  of  bisulphide  of  carbon.  One  face 
of  the  prism  is  protected  by  a  diaphragm 
with  a  longitudinal  slit,  through  which  the 
beam  passes  into  the  prism.  It  emerges  de- 
composed at  the  other  side.  I  permit  the 
colors  to  pass  through  a  cylindrical  lens, 
which  so  squeezes  them  together  as  to  pro- 
duce upon  the  screen  a  sharply-defined  rect- 
angular image  of  the  longitudinal  slit.  In 
that  image  the  colors  are  re-blended,  and  you 
see  it  perfectly  white.  Between  the  prism 
and  the  cylindrical  lens  may  be  seen  the 
colors  tracking  themselves  through  the  dust 
Df  the  room.  Cutting  off  the  more  refrangi- 
ble fringe  by  u  card,  the  rectangle  is  seen 
red  ;  cutting  off  the  less  refrangible  fringe, 
the  rectangle  is  seen  blue.  By  means  of  a 
thin  glass  prism,  I  deflect  one  portion  of  the 
colors,  and  leave  the  residual  portion.  On 
the  screen  are  now  two  colored  rectangles 
produced  in  this  way.  These  are  comple- 
mentary colors — colors  which,  by  their  union, 
produce  white.  Note  that,  by  judicious 
management,  one  of  these  colors  is  icndered 
yellow,  and  the  other  blue.  I  withdraw  the 
thin  prism  ;  yellow  falls  upon  blue,  and  we 
have  white  as  the  result  of  their  union.  On 
our  way,  we  thus  abolish  the  fallacy  first 
exposed  by  Helmholtz,  that  the  mixture  of 
blue  and  yellow  lights  produces  green. 

Again,  restoring  the  circular  aperture,  we 
obtain  once  more  a  spectrum  like  that  of 
Newton.  By  means  of  a  lens,  we  gather  up 
these  colors,  and  build  them  together  not  to 
an  image  of  the  aperture,  but  to  an  image  of 
the  carbon  points  themselves.  Finally,  in 
virtue  of  the  persistence  of  impressions  upon 
the  retina,  by  means  of  a  rotating  disk,  on 
which  are  spread  in  sectors  the  colors  of  the 
spectrum,  we  blend  together  the  prismatic 
colors  in  the  eye  itself,  and  thus  produce  the 
impression  of  whiteness. 

Having  unravelled  the  interwoven  con- 
stituents  of  white  light,  we  have  next  to 
inquire,  What  part  the  constitution  s:> 
revealed  enables  this  agent  to  play  in  Nature  ? 


To  it  we  owe  all  the  phenomena  of  color; 
and  yet  not  to  it  alone,  for  there  must  be  a 
certain  relationship  between  the  ultimate  par- 
ticles of  natural  bodies  and  light  to  enable 
them  to  extract  from  it  the  luxuries  of  color. 
But  the  function  of  natural  bodies  is  here 
selective,  not  creative.  There  is  no  color  gen- 
erated by  any  natural  body  whatever  Natural 
bodies  have  showered  upon  them,  in  the 
white  light  of  the  sun,  the  sum  total  of  all 
possible  colors,  and  their  action  is  limited  to 
the  sifting  of  that  total,  the  appropriating 
from  it  of  the  colors  which  really  belong  to 
them,  and  the  rejecting  of  those  which  do 
not.  It  will  fix  this  subject  in  your  minds  if 
I  say  that  it  is  the  portion  of  light  which 
they  reject,  and  not  that  which  belongs  to 
them,  that  gives  bodies  their  colors. 

Let  us  begin  our  experimental  inquiries  here 
by  asking,  \Vhatisthemeaningof  blackness? 
Pass  a  black  ribbon  in  succession  through  the 
colors   of    the   spectium ;    it   quenches    all. 
This  is  the  meaning  of  blackness — it  is  the 
result  of  the  absorption  of  all  the  constituents 
of  solar  light.     Pass  a  red  r'.bbon  through  the^ 
spectrum.       In  the  red  light  the  ribbon  is  a 
vivid   red.     "Why  ?     Because   the  light   that 
enters  the  ribbon  is  not  quenched  or  absorbed, 
but  sent  back  to  the  eye.     Place  the  same  rib. 
bon  in  the  green  or  blue  of  the  spectrum  ;  \fe 
is  black  as  jet.     It.  absorbs   the   green  ai/4. 
blue  light,  and  leaves  the  s pace  on  which  they 
fall   a   space   of   intense  darkness.     Place  a. 
green  ribbon  in  the  green  of  the  spectrum. 
It  shines  vividly  with  its  proper  color  ;  transfei;- 
it  to  the  red,  it  is  black  as  jet.     Here  it  ab- 
sorbs all  the  light  that  falls  upon  it,  and  offers , 
mere  darkness  to  the  eye.     Whert  white  light 
is  employed,  the  red  sifts  it  by  quenching  the 
green,  and  the  green  sifts  it  by  quenching  the 
red,  both  exhibiting  the  residual  color.     Thus 
the  process  through  which  natural  bodies  ac- 
quire  their   colors  is  a  negative  one.       The 
colors  are  produced  by  subtraction,  not   by 
addition.     This  red  glass  is ,  red  because  it 
destroys  all  the  more  refrangible  rays  of   the  • 
spectrum.    This  blue  liquid  is  blu,e^ because  it, 
destroys  all  the  less  refrangible  rays.     Both 
together  are  opaque  because  the  light  trans- 
mitted by  the  one  is  quenched  by  the  other. 
In  this  way  by  the  union  of  two  transparent, 
substances  we  obtain  a  combination  as  dark 
as  pitch  to   solar  li^ht.     This    other   liquid, 
finally  is  purple  because  it  destroys  the  gree.i 
and  the  yellow,  and  allows  the  terminal  colors  . 
of  the  Lpectrum  to  pass  unimpeded.     From 
thp  blending  of  the  blue  and  the  red  this  gor- 
geous color  is  produced. 

These  experiments  prepare  us  for  the  fur-, 
ther  consideration  of  a  point  already  adverted 
to,  and  regarding  which  error  has  found  cur- 
rency for  ages.  You  will  find  it  stated  in 
books  that  blue  and  yellow  lights  mixed  to- 
gether produce  green.  But  blue  and  yellow 
nave  been  just  proved  to  be  complementary 
colors,  producing  white  by  their  mixture. 
The  mixture  of  blue  and  yellow  pigments  un- 


SIX  LECTURES  ON  LIGHT. 


doubtedly  produces  green,  but  the  mixture  of 
pigments  is  totally  different  from  the  mixture 
of  lights.  Helmholtz,  who  first  proved  yel- 
low and  blue  to  complementary  ..colors,  has 
revealed  the  cause  of  the  green  in  the  case  of 
the  pigments.  No  natural  color  is  pure.  A 
blue  liquid  or  a  blue  powder  permits  not  only 
the  blue  to  pass  through  it,  but  a  portion  of 
the  adjacent  green.  A  yellow  powder  is 
transparent  not  only  to  the  yellow  light,  but 
also  in  part  transparent  to  the  adjacent  green. 
Now,  when  blue  and  yellow  are  mixed  to- 
gether, the  blue  cuts  off  the  yellow,  the  orange, 
and  the  red  ;  the  yellow,  on  the  other  hand, 
cuts  off  the  violet,  the  indigo,  and  the  blue. 
Green  is  the  only  color  to  which  both  are 
transparent,  and  the  consequence  is  that, 
when  white  light  falls  upon  a  mixture  of  yel- 
low and  blue  powders,  the  green  alone  is  sent 
back  to  the  eye.  I  have  already  shown  you 
that  the  fine  blue  ammonia-sulphate  of  copper 
transmits  a  large  portion  of  green,  while  cut- 
ting off  all  the  less  refrangible  light.  A  yel- 
low solution  of  picric  acid  also  allows  the 
green  to  pass,  but  quenches  all  the  more  re- 
frangible light.  What  must  occur  when  we 
send  a  beam  through  both  liquids  ?  The 
green  band  of  the  spectrum  alone  remains 
upon  the  screen. 

This  question  of  absorption  is  one  of  the 
most  subtle  and  difficult  in  molecular  physics. 
Vvc  are  not  yet  in  a  condition  to  grapple  with 
:£„  but  we  shall  be  by-and-by.  Meanwhile, 
"'we  may  profitably  glance  back  on  the  web  of 
jwtlations  which  these  experiments  reveal  to 
its.  We  have,  in  the  first  place,  in  solar 
light  an  agent  of  exceeding  complexity,  com- 
posed of  innumerable  constituents,  refrangi- 
ble in  different  degrees.  We  find,  secondly, 
tike  atoms  and  molecules  of  bodies  gifted 
writ  la  the  power  of  sifting  solar  light  in  the 
most  various  ways,  and  producing  by  this 
•ifting  the  colors  observed  in  nature  and  art. 
To  do  this  they  must  possess  a  molecular  j 
'Structure  commensurate  in  complexity  with  j 
that  of  light  itself.  Thirdly,  we  have  :he  j 
.human  eye  and  brain  so  organized  as  to  be 
•able  to  take  in  and  distinguish  the  multitude 
•Oif  impressions  thus  generated.  Thus,  the 
light  .at  starting  is  complex;  to  sift  and  select 
ic  as  they  do  natural  bodies  must  be  complex. 
Fiaally,  to  take  in  the  impressions  thus  gen- 
vcjated,  the  human  eye  and  brain  must  be 
iiighly  complex.  Whence  this  triple  com 
p.txiy?  If  what  are  called  material  pur- 
pios^s  were  the  only  end  to  be  served,  a  much 
simpler  mechanism  would  be  sufficient.  But, 
instead  of  simplicity — instead  of  the  princi- 
ple of  parsimony — we  have  prodigality  of  re- 
lation and  adaptation,  and  this  apparently 
for  the  sole  purpose  of  enabling  us  to  see 
things  robed  in  the  splendor  of  color.  Would 
it  not  seem  that  Nature  harbored  the  inten- 
tion <<if  educating  us  for  other  enjoyments 
than  those  derivable  from  meat  and  drink  ? 
At  all  events,  whatever  Nature  meant — and 
k  would, be. mere  ..pres.  mption  to  dogmatize 


as  to  what  she  meant — we  find  ourselves  here 
as  the  issue  and  upshot  of  her  operations,  en- 
dowed with  capacities  to  enjov  not  only  the 
materially  useful,  but  endowed  with  others  of 
indefinite  scope  and  application,  which  deal 
alone  with  the  beautiful  and  the  true. 


LECTURE  II. 

Origin  of  Physical  Theories :  Scope  of  the  Imagina- 
tion :  Newton  and  the  Emission  Theory:  Verifica- 
tion of  Physical  Theories:  The  Luminiferous ; 
Ether:  Wave-Theory  of  Light:  Thomas  Young: 
Fresnel  and  Arago  :  Conceptions  of  Wave-Motion: 
Interference  of  Wavts;  Constitution  of  Sound- 
Waves:  Analogies  of  Sound  and  Light:  Illustra- 
tions of  Wave-Motion:  Interference  of  Sound- 
Waves :  Optical  Illustrations:  Pitch  and  Color: 
Lengths  of  the  Waves  of  Light  and  Kates  of 
Vibration  of  the  Ether- Particles:  Interference  of 
Light :  Phenomena  which  first  suggested  the  Un- 
dulatory  Theory:  Hooke  and  the  .Colors  of  Thin 
Plates:  The  Soip-Bubble:  Newton's  Rings: 
Theory  of  lv  Fits:  "  Its  Explanation  of  the  Kings  : 
Overthrow  of  the  Tneory :  Colors  of  Mother-uf- 
Pearl. 

WE  might  vary  and  extend  our  experi- 
ments on  light  indefinitely,  and  they  cer- 
tainly would  prove  us  to  possess  a  wonderful 
mastery  over  the  phenomena.  But  the  vts- 
ture  of  the  agent  only  would  thus  be  re- 
vealed, not  the  agent  itself.  The  human 
mind,  however,  is  so  constituted  and  so  edu- 
cated as  regards  natural  things,  that  it  can 
never  rest  satisfied  with  this  outward  view  of 
them.  Brightness  and  freshness  take  pos- 
session of  the  mind  when  it  is  crossed  by  the 
light  of  principles,  which  show  the  facts  of 
Nature  to  be  organically  connected. 

Let  us,  then,  inquire  what  this  thing  is 
that  we  have  been  generating,  reflecting,  re- 
fracting, and  analyzing. 

In  doing  this,  we  shall  learn  that  the  life 
of  the  experimental  philosopher  is  twolold. 
He  lives,  in  his  vocation,  a  life  of  the  senses, 
using  his  hands,  eyes,  and  ears  in  his  experi- 
ments, but  such  a  question  as  that  now  before 
us  carries  him  beyond  the  margin  of  the 
senses.  He  cannot  consider,  much  less  an- 
swer, the  question,  "What  is  light?"  with- 
out transporting  himself  to  a  world  which 
undelies  the  sensible  one,  and  out  of  wnich, 
in  accordance  with  rigid  law,  all  optical  phe- 
nomena spring.  To  realize  this  subsensible 
world,  if  1  may  use  the  term,  the  mit..d  must 
possess  a  certain  pictorial  power.  It  has  to 
visualize  the  invisible.  It  must  be  able  to 
form  definite  images  of  the  things  which  that 
subsensib'e  world  contains  ;  and  to  say  that, 
if  such  or  such  a  state  of  things  exist  in  that 
world,  then  the  phenomena  which  appear  in 
ours  must,  of  necessity,  grow  out  of  this 
state  of  things.  If  the  picture  be  correct, 
the  phenomena  are  accounted  for  ;  a  physical 
theory  has  been  enunciated  which  unites  and 
explains  them  all. 

This  conception  of  physical  theory  implies, 
as  you  perceive,  the  exercise  of  the  imagina- 
tion. Do  not  be  afraid  of  this  word,  which 
seems  to  rentier  so  many  respectable  people, 


SIX  LECTURES  ON  LIGHT. 


both  m  the  ranks  of  science  and  out  of  them,    of  elastic  collision.      The  fact  of  optical  re- 


uncomfortable.     That  men  in  the  ranks  of 
science  should  feel  thus  is,  I  think,  a  proof 


flection  certainly  occurred  as  if  light  consist- 
ed of  elastic  particles,  and  this  was  Newton's 


that  they  have  suffered  themselves  to  be  mis-  ( sole  justification  for  introducing  them, 
led  by  the  popular  definition  of  a  great  (  But  this  is  not  all.  In  another  important 
faculty  instead  of  observing  its  operation  in  |  particular,  also.  Newton's  conceptions  re- 
their  own  minds.  Without  imagination  we  garding  the  nature  of  light  were  influenced 

by  his  previous  knowledge.  He  had  been 
working  at  the  phenomena  of  eravitation. 
and  had  made  himself  at  home  amid  the  oper- 
ations of  this  universal  power.  Perhaps  hrs 
mind  at  this  time  was  too  freshly  and  too 
deeply  imbued  with  these  notions  to  permit 
of  his  forming  an  unfettered  judgment  re- 


imagmation 

cannot  take  a  step  beyond  the  bourne  of  the 
mere  animal  world,  perhaps  not  even  to  the 
edge  of  this.  But,  in  speaking  thus  of 
imagination,  I  do  not  mean  a  riotous  power 
which  deals  capriciously  with  facts,  but  a 
well-ordered  and  disciplined  power,  whose 
sole  function  is  to  form  conceptions  which 
the  intellect  imperatively  demands.  Imagina- 
tion thus  exercised  never  really  severs  itself 
from  the  world  of  fact.  This  is  the  store- 
house from  which  all  its  pictures  are  drawn  ; 
and  the  magic  of  its  art  consists,  not  in 
creating  things  anew,  but  in  so  changing  the 
magnitude,  position,  and  other  relations  of 
sensible  things,  as  to  render  them  fit  for  the 
Requirements  of  the  intellect  in  the  subsen- 
uible  world.* 

I  will  take,  as  an  illustration  of  this  sub- 
ject, the  case  of  Newton.  Before  he  began 
to  deal  with  light,  he  was  intimately  ac- 
quainted with  the  laws  of  elastic  collision, 
which  all  of  you  have  seen  more  or  less  per- 
fectly illustrated  on  a  billiard-table.  As  re- 
gards the  collision  of  sens:ble  masses,  New- 
ton knew  the  angle  of  incidence  to  be  equal 
to  the  angle  of  reflection,  and  he  also  knew 
that  experiment,  as  shown  in  our  last  lecture, 
had  established  the  same  law  with  regard  to 
light.  He  thus  found  in  his  previous  knowl- 
edge the  mateiial  for  theoretic  images.  He 
had  only  to  change  the  magnitude  of  concep- 
tions already  in  his  mind  to  arrive  at  the 
Emission  Theory  of  Light.  He  supposed 
light  to  consist  of  elastic  particles  of  incon- 
ceivable minuteness  shot  out  with  inconceiv- 
able rapidity  by  luminous  bodies.  Such  par- 
ticles impinging  upon  smooth  surfaces  were 
reflected  in  accordance  with  the  ordinary  law 


*  The  following  charming  extract,  bearing  upon 
this  point,  was  discovered  and  written  out  for  me  by 
my  friend,  Dr.  Bence  Jones,  Hon.  Secretary  to  the 
Royal  Institution  • 

"  In  every  kind  of  magnitude  there  is  a  degree  or 
'sort  to  which  our  sense  is  proportioned,  the  percep- 
tion and  knowledge  cf  which  is  of  the  greatest  use  to 
mankind.  The  same  is  the  groundwork  of  philoso- 
phy :  for,  though  all  sorts  and  degrees  are  equally 
the  object  of  philosophical  speculation,  yet  it  is  from 
those  which  are  proportioned  to  sense  that  a  philoso- 
pher must  set  out  in  his  inquiries,  ascending  or  de- 
scending afterwards,  as  his  pursuits  may  require.  He 


garding  the  nature  of  light.  Be  that 
as  it  may,  Newton  saw  in  refraction 
the  action  of  an  attractive  force  exerted 
on  the  light-particles.  He  carried  his 
conception  out  with  the  most  severe  con- 
sistency. Dropping  vertically  downwards 
towards  the  earth's  surface,  the  motion  of  a 
body  is  accelerated  as  it  approaches  the  earth. 
Dropping  in  the  same  manner  downwards  on 
a  horizontal  surface,  say  through  air  on  glass 
or  water,  the  velocity  of  the  light-particles, 
when  they  come  close  to  th2  surface,  was, 
according  to  Newton,  also  accelerated.  Ap- 
proaching such  a  surface  obliquely,  he  sup- 
posed the  particles,  when  close  to  it,  to  be 
drawn  down  upon  it,  as  a  projectile  is 
drawn  by  gravity  to  the  surface  of  the  earth. 
This  deflection  was,  according  to  Newton, 
the  refraction  seen  in  our  last  lecture.  Final- 
ly, it  was  supposed  that  differences  of  color 
might  be  due  to  differences  in  the  sizes  of  the 
particles.  This  was  the  physical  theory  of 
light  enunciated  and  defended  by  Newton; 
and  you  will  observe  that  it  simply  consists 
in  the  transference  of  conceptions  born  in  the 
world  of  the  senses  to  a  substnsible  world. 

But,  though  the  region  of  physical  theory 
lies  thus  behind  the  world  of  senses,  the  veri- 
fications of  theory  occur  in  that  world.  Lay- 
ing the  theoretic  conception  at  the  root  of 
matters,  we  determine  by  rigid  deduction 
what  are  the  phenomena  which  must  of  neces- 
sity grow  out  of  this  root.  If  the  phenomena 
thus  deduced  agree  with  those  of  the  actual 
world,  it  is  a  presumption  in  favor  of  the 
theory.  If  as  new  classes  of  phenomena  arise 
they  also  are  found  to  harmonize  with  theo- 
retic deduction,  the  presumption  becomes  still 
stronger.  If,  finally,  the  theory  confers  pro- 
phetic vision  upon  the  investigator,  enabling 
him  to  predict  the  existence  of  phenomena 
which  have  never  yet  been  seen,  and  if  those 


does  well  indeed  to  take  his  views  from  many  points  |  predictions  be  found  on  trial  to  be  rigidly  cor- 
rect, the  persuasion  of  the  truth  of  the  theory 
becomes  overpowering.  Thus  working  back- 
wards from  a  limited  number  of  phenomena, 
genius,  by  its  own  expansive  force,  reaches  a 
conception  which  covers  all  the  phenomena. 
There  is  no  more  wonderful  performance  of 
the  intellect  than  this.  And  we  can  render 
no  account  of  ii.  Like  the  scriptural  gift  of 
the  Spirit,  no  man  can  tell  whence  it  cometh. 
The  passage  from  fact  to  principle  is  some- 


of  sight,  and  supply  the  defects  of  sense  by  a  well- 
regulated  imagination  ;  nor  is  he  to  be  confined  by 
any  limit  in  space  or  time  ;  but,  as  his  knowledge  of 
Nature  is  founded  on  the  observation  of  sensible 
things,  he  must  begin  with  these,  and  must  often  re- 
turn to  them  to  examine  his  progress  by  them. 
Here  is  his  secure  hold ;  and  as  he  sets  out  from 
thence,  so  if  he  likewise  trace  not  often  his  steps 
backwards  with  caution,  he  will  be  m  hazard  of  losing 
ki»  way  in  the  labyrinths  of  Nature." — (Maclaurin  : 
/in  Account  of  Sir  I.  Neivtoris  Philosophical  Dis- 
coveries. Written  1728  ;  sttcond  edition,  1750  ;  pp. 


10 


SIX  LECTURES  ON  LIGHT. 


times  slow,  sometimes  rapid,  and  at  all  times 
a  source  of  intellectual  joy.  When  rapid,  the 
pleasure  is  concentrated  and  becomes  a  kind 
of  ecstasy  or  intoxication.  To  any  one  who 
has  experienced  this  pleasure,  even  in  a  mod- 
erate degree,  the  action  of  Archimedes  when 
he  quitted  the  bath,  and  ran  naked,  crying 
"  Eureka!  "  through  the  streets  of  Syracuse, 
becomes  intelligible. 

How,  then,  did  it  fare  with  the  theory  cf 
Newton,  when  the  deductions  from  it  were 
brought  face  to  face  with  natural  phenomena  ? 
To  the  mind's  eye,  Newton's  elastic  particles 
present  themselves  like  particles  of  sensible 
magnitude.  The  same  reasoning  applies  to 
both  ;  the  same  experimental  checks  exist  for 
both.  Tested  by  experiment,  then,  Newton's 
theory  was  found  competent  to  explain  many 
facts,  and  with  transcendent  ingenuity  its 
author  sought  to  make  it  account  for  all.  He 
so  far  succeeded,  that  men  so  celebrated  as 
Laplace  and  Malus,  who  lived  till  1812,  and 
13iot  and  Brewster,  who  lived  till  our  own 
lime,  were  found  among  his  disciples. 

Still,  even  at  an  early  period  of  the  existence 
of  the  Emission  Theory,  one  or  two  great 
names  were  found  recording  a  protest  against 
it  ;  and  they  furnish  another  illustration  of 
the  law  that,  in  forming  theories,  the  scientific 
imagination  must  draw  its  materials  from  the 
world  of  fact  and  experience.  It  was  known 
long  ago  that ;  ound  is  conveyed  in  waves  or 
puses  through  the  air  ;  and  no  sooner  was  this 
truth  well  housed  in  the  mind  than  it  was  trans- 
formed into  a  theoretic  conception.  It  was 
supposed  that  light,  like  sound,  might  also  be 
the  product  of  wave-motion.  But  what,  in 
this  case,  could  be  the  material  forming  the 
waves  ?  For  the  waves  of  sound  we  have  the 
air  cf  our  atmospheie  ;  but  the  stretch  of  im- 
agination which  filled  all  space  with  a  luminif- 
erous  ether  trembling  with  the  waves  of  light 
was  so  bold  as  to  shock  cautious  minds.  In 
one  of  my  latest  conversations  with  Sir  David 
Brewster  he  said  to  me  that  his  chief  objection 
to  the  undulatory  theory  of  light  was  that  he 
could  not  think  the  Creator  guilty  of  so  clumsy 
a  contrivance  as  the  filling  of  space  with  ether 
in  order  to  produce  light.  This,  I  may  say, 
is  very  dangerous  ground,  and  the  quarrel  of 
science  with  Sir  David,  on  this  point,  as  with 
many  other  persons  on  other  points,  is,  that 
they  profess  to  know  too  much  about  the 
mind  of  the  Creator. 

This  conception  of  an  ether  was  advocated 
and  indeed  applied  to  various  phenomena  of 
optics  by  the  celebrated  astronomer,  Huy- 
gh^ns.  It  was  espoused  and  defended  by  the 
celebrated  mathematician,  Euler.  They  were, 
however,  opposed  by  Newton,  whose  authority 
at  -lie  time  bore  them  down.  Or  shall  I  say 
it  was  authority  merely  ?  Not  quite  so. 
Newton's  preponderance  was  in  some  degree 
due  to  the  fact  that,  though  IIuyghe::s  and 
Euler  were  right  in  the  main,  t..ey  did  not 
oossess  sufficient  data  to  prove  themselves 
right.  No  human  authority,  however  high, 


can  maintain  itself  against  the  voice  of  Nature 
speaking  through  experiment.  But  the  voice 
of  Nature  may  be  an  uncertain  voice,  through 
the  scantiness  of  data.  This  was  the  case  at 
the  period  now  referred  to,  and  at  such  a  pe- 
riod by  the  authority  of  Newton  all  antago- 
nists were  naturally  overborne. 

Still,  this  great  Emission  Theory,  which 
held  its  ground  so  long,  resembled  one  of 
those  circles  which,  according  to  your  coun- 
tryman Emerson,  the  force  of  genius  periodi- 
cally draws  round  the  operations  of  the  in- 
tellect, but  which  are  eventually  broken 
through  by  p  es^ure  from  behind.  In  the 
year  1773  was  born,  at  Milverton,  in  Somer- 
setshire, one  of  the  most  remarkable  men  that 
England  ever  produced.  He  was  educated 
for  the  profession  of  a  physician,  but  was  too 
strong  to  be  tied  down  to  professional  routine. 
He  devoted  himself  to  the  study  of  natural 
philosophy,  and  became  in  all  its  departments 
a  master.  He  was  also  a  master  of  letters. 
Languages,  ancient  and  modern,  were  housed 
within  his  brain,  and,  to  use  the  words  of  his 
epitaph,  "he  first  penetrated  the  obscurity 
which  had  veiled  for  ages  the  hieroglyphics  of 
Egypt."  It  fell  to  the  lot  of  this  man  to  dis- 
cover facts  in  optics  which  Newton's  theory 
was  incompetent  to  explain,  and  his  mind 
roamed  i*  search  of  a  sufficient  theory.  He 
had  made  himself  acquainted  with  all  the 
phenomena  of  wave-moti  ,n  ;  with  all  the 
phenomena  of  sound  ;  working  successfully 
in  this  domain  as  an  original  discoverer. 
Thus  informed  and  disciplined,  he  was  pre- 
pared to  detect  any  resemblance  which  might 
reveal  itself  between  the  phenomena  of  light 
and  those  of  wave-motion.  Such  resem- 
blances he  did  detect  ;  and,  spurred  on  by 
the  discovery,  he  pursued  his  speculations  and 
his  experiments,  until  he  finally  succeeded  in 
placing  on  an  immovable  basis  the  Undulatory 
Theory  of  Light. 

The  founder  of  this  great  theory  "yas 
Thomas  Young,  a  name,  perhaps,  unfamiliar 
to  many  of  you.  Permit  me,  by  a  kind  of 
geometrical  construction  which  I  once  em- 
ployed in  London,  to  give  you  a  notion  of  the 
magnitude  of  this  man.  Let  Newton  stand 
erect  in  his  age,  and  Young  in  his.  Draw  a 
straight  line  from  Newton  to  Youn^,,  which 
shall  form  a  tangent  to  the  heads  of  both. 
This  line  would  slope  downwards  from  New- 
ton to  Young,  because  Newton  was  certainly  • 
the  taller  man  of  the  two.  But  the  slope 
would  not  be  steep,  for  the  difference  of  stat- 
ure was  not  excessive.  The  line  would  form 
what  engineers  call  a  gentle  gradient  from 
Newton  to  Young.  Place  underneath  this 
line  the  biggest  man  born  in  the  interval 
between  both.  Pie  would  not,  in  my  opinion, 
reach  the  line  ;  for  if  he  did  he  would  be 
taller  intellectually  than  Young,  and  there 
was,  I  believe,  none  taller.  But  I  do  not 
want  you  to  rest  on  English  estimates  of 
Young;  the  German,  Helmholtz,  a  kindred 
jenius,  thus  speaks  of  him  :  "  His  was  one 


SIX  LECTURES  ON  LIGHT. 


of  the  most  profound  minds  that  the  world  j  which  ?.t  any  moment  constitute  the  wave, 

Stand  upon  the  sea-shore  and  observe  ihe 


has  ever  seen  ;  but  he  had  the  misfortune  to 
be  too  much  in  advance  of  his  age.  He  ex- 
cited the  wonder  of  his  contemporaries,  who, 
however,  were  unab'e  to  follow  him  to  the 
heights  at  which  his  daring  intellect  was 

accustomed  to  soar.  His  most  important  j  that  every  particle  of  water  along  the  front 
ide  s  lay,  therefore,  buried  and  forgotten  in  of  the  wave  is  in  the  act  of  rising,  while 
'the  folios  of  the  Royal  Society,  until  a  new  every  particle  along  its  back  is  in  the  act  of 
generation  gradually  and  painfully  made  t^e  |  sinking.  The  particles  in  front  reach  in  suc- 
same  discoveries,  and  proved  the  exactness  j  cession  the  crest  of  the  wave,  and  as  soon  as 


advancing  rollers  before  they  are  distorted  by 
the  friction  of  the  bottom.  Every  wave  has 
a  back  and  a  front,  and,  if  you  clearly  seiae 
the  image  of  the  moving  wave,  you  will  see 


of  his  assertions  and  the  truth  of  his  demon- 
strations." 

It  is  quite  true,  as  Helmholtz  says,  that 
Aroung  was  in  advance  of  his  age  ;  but  some- 
thing is  to  be  added  which  illustrates  the 
responsibility  of  our  public  writers.  Foi 
twenty  years  this  man  of  genius  was  quenched 
— hidden  from  the  appreciative  intellect  of 
his  countrymen — deemed  in  fact  a  creamer, 
liirough  the  vigorous  audacity  of  a  writer 
who  had  then  possession  of  the  public  ear, 
and  who  in  the  Edinburgh  Review  pourtd 
ridicule  upon  Young  and  his  speculations. 
To  the  celebrated  Frenchmen,  Fresnel  and 
Arago,  lie  was  first  indebted  for  the  restitu- 
tion of  hu  rights,  for  they,  especially  Fresnel, 
remade  independently,  as  Helmholtz  says, 
and  vastly  extended  l.is  discoveries.  To  the 


the  crest  is  passed  they  begin  to  fall.  They 
then  reac  i  the  furrow  or  sinus  of  the  wave, 
and  can  sink  no  farther.  Immediately  after- 
wards they  become  the  front  of  the  succeed- 
ing wave,  rise  again  until  they  reach  the 
crest,  and  then  sink  as  before.  Thus,  while 
the  waves  pass  onward  horizontally,  the 
individual  particles  are  simply  lifted  up  and 
down  vertically.  Observe  a  sea-fow],  or,  if 
you  are  a  swimmer,  abandon  yourself  to  the 
action  cf  the  waves  ;  you  are  not  carried  for- 
ward, but  simply  rocked  up  and  down.  The 
propagation  of  a  wave  is  the  propagation  of 
a  form,  and  not  the  transference  of  the  sub- 
stance which  constitutes  the  wave. 

The  length  of  the  wave  is  the  d  stance 
from  crest  to  crest,  while  the  distance  through 
which  the  individual  particles  oscillate  is 


students  cf  his  works  Young  has  long  since  j  called  the  amplitude  of  the  oscillation.     You 
appeared  in  his  true  light,  but  these  twenty    will  notice  that  in  this  description  the  parti- 
cles of  water  are   made  to  vibrate  across  the 
line  of  propagation.* 


blank  years  pushed  him  from  the  public 
mind,  which  became  in  turn  filled  with  the 
fame  of  Young's  colleague  at  the  Royal  In 
stitution,  Davy,  and  afterwards  with  the 
fame  of  Faraday.  Carlyle  refers  to  the  re- 
mark of  Novalis,  that  a  man's  self-trust  is 
enormously  increased  the  moment  he  finds 
that  others  believe  him.  If  the  opposite 
remark  be  true — if  it  be  a  fact  that  public 
disbelief  weakens  a  man's  force — there  is  no 
calculating  the  amount  of  damage  these 
twenty  years  of  neglect  may  have  done  to 
Young's  productiveness  as  an  investigator. 
It  remains  to  be  s:ated  thai  his  assailant  was 
Mr.  Henry  Brougham,  afterwards  Lord 
Chancellor  of  England. 

Our  hardest  woik  is  now  before  us.  And, 
as  I  have  often  had  occasion  to  notice  that 
capacity  for  hard  work  depends  in  a  great 
measure  on  the  antecedent  winding  up  of  the 
will  and  determination,  I  would  call  upon 
you  to  gird  up  your  loins  for  our  coming 
labors.  If  we  succeed  in  climbing  the  hill 
which  faces  us  to-night,  our  future  efforts 
will  be  comparatively  light. 

In  the  earliest  writings  of  the  ancients  we 
find  the  notion  that  sound  is  conveyed  by  the 
air.  Aristotle  gives  expression  to  this  no- 
tion, and  the  great  architect  Vitruvius  com- 
pares the  waves  of  sound  to  waves  of  water. 
But  the  real  mechanism  of  wave-motion  was  i 
hidden  from  the  ancitnts,  and  indeed  was 
not  mad-  clear  until  the  time  of  Newton. 
The  a  ntral  difficulty  of  the  subject  was,  to 
distinguish  between  the  motion  of  the  wave 
itself  and  the  motion  of  the  particles 


And  now  we  have  to  take  a  step  forward, 
and  it  is  the  most  important,  step  of  ail.  You 
can  picture  two  serie:  of  waves  proceeding 
from  different  origins  through  the  same 
water.  When,  for  example,  you  throw  two 
stones  into  still  water,  the  ring- waves  pro- 
ceeding from  the  two  centres  of  disturbance 
intersect  each  other.  Now,  no  mailer  how 
numerous  these  waves  may  be.  the  law  holds 
good  that  the  mo  lion  of  every  particle  of  the 
water  is  the  algebraic  sum  of  a.l  the  motions 
imparted  to  it.  If  crest  coincide  with  crest, 
the  wave  is  lifted  to  a  double  height;  if  fur- 
row coincide  with  crest,  the  motions  are  in 
opposition,  and  thei.  sum  ij  zero.  We  have 
then  still  water,  which  we  shall  learn  pres- 
ently corresponds  to  what  we  call  darkness  in 


reference  to  our  present  subjs 


This  action 


of  wave  upon  wave  is  technically  called  in- 
terference, a  term  to  be  remembered. 

Thomas  Young's  fundamental  discovery  in 
optics  was  that  the  principle  of  Interference 
applied  to  light.  Long  prior  to  his  timCj  an 
Italian  philosopher.  Giimaldi,  had  stated 
that,  under  certain  circumstances,  two  thin 
beams  of  light,  each  of  which,  acting  singly, 
produced  a  luminous  spot  upon  a  wnite  wall, 
when  caused  to  act  together,  partially 


*  I  do  not  wish  to  encumber  the  conception  here 
with  the  details  of  the  motion,  but  I  inay  draw  atten- 
tion to  the  beautiful  model  of  Professor  Lyinao, 
wherein  waves  are  shown  10  be  produc.ed  by  tho  cir- 
cular motion  if  the -particles.  This,  as  uroved  by 
the  brothers  Weber,  is  the  real  motley  •.•:  :~e  case  uf 
water-waves. 


12 


SIX  LECTURES  ON  LIGHT. 


quenched  each  other  and  darkened  the  spot. 
This  was  a  statement  of  fundamental  signifi- 
cance, but  it  required  the  discoveries  and  the 
genius  of  Young  to  give  it  meaning.  How 
he  did  so,  I  will  now  try  to  make  clear  to 
you.  You  know  that  air  is  compressible  ; 
that  by  pressure  it  can  be  rendered  more 
dense,  and  that  by  dilatation  it  can  be  ren- 
dered more  rare.  Properly  agitated,  a  tun- 
ing-fork now  sounds  in  a  manner  audible  to 
you  all,  and  most  of  you  know  that  the  air 
through  which  the  sound  is  passing  is  par- 
celled Out  into  spaces  in  which  the  air  is  con- 
densed, followed  by  other  spaces  in  which 
the  air  is  rarefied.  These  condensations  and 
rarefactions  constitute  what  we  call  waves  of 
sound.  You  can  imagine  the  air  of  a  room 
traversed  by  u  series  of  such  waves,  and  you 
can  imagine  a  second  series  sent  through  the 
same  air,  and  so  related  to  the  first  that  con- 
densation coincides  with  condensation  and 
rarefaction  with  rarefaction.  The  conse- 
quence of  this  coincidence  would  be  a  louder 
sound  than  that  produced  by  either  system  of 
waves  taken  singly.  But  you  can  also  ima- 
gine a  state  of  things  where  the  condensa- 
tions of  the  one  system  fall  upon  the  rarefac- 
tions of  the  other  system.  In  this  case  th: 
two  systems  would  completely  neutralize  each 
other.  Each  of  them,  taken  singly,  produces 
sound;  both  of  them,  taken  together,  pro- 
duce no  sound.  Thus,  by  adding  sound  to 
sound  we  produce  silence,  as  Grimaldi  in  his 
experiment  produced  darkness  by  adding 
light  to  light. 

The  analogy  between  sound  and  light  here 
at  once  flashes  upon  the  mind.  Young  gen 
eralized  this  observation.  He  discovered  a 
multitude  of  similar  cases,  and  determined 
their  precise  conditions.  On  the  assumption 
that  light  was  wave-motion,  all  his  experi- 
ments on  interference  were  explained  ;  on  the 
assumption  that  light  wai  flying  particles, 
nothing  was  explained.  In  the  time  of  Huy- 
ghens  and  Euler  a  medium  had  been  assumed 
tor  the  transmission  of  the  waves  of  light  ; 
but  Newton  raised  the  objection  that,  if  light 
consisted  of  the  waves  of  such  a  medium,  ' 
shadows  could  not  exist.  The  waves,  he 
contended,  would  bend  round  opaque  bodies 
and  produce  the  motion  of  light  behind  them, 
as  sound  turns  a  corner,  or  as  waves  of  water 
wash  round  a  rock.  It  was  proved  that  the 
bending  round  referred  to  by  Newton  actually 
occurs,  but  that  the  inflected  waves  abolish 
each  other  by  their  mutual  interference. 
Young  also  discerned  a  fundamental  differ- 
ence between  ti'.e  waves  of  light  and  those  of 
sound.  Could  you  see  the  air  through  which 
sound-waves  are  passing,  you  would  observe 
every  individual  particle  of  air  oscillating  to 
fcnd  fro  in  the  direction  of  propagation. 
Could  you  see  the  ether,  you  would  also  find 
every  individual  particle  making  a  small  ex- 
cursion to  and  fro,  but  here  the  motion,  like 
that  assigned  to  the  water-particles  above  re- 
ferred to,  would  be  across  the  line  of  propa- 


gation. The  vibrations  of  the  air  are  longi- 
tudinal, the  vibrations  of  the  ether  are  trans- 
versal. 

It  is  my  desire  that  you  should  realize  with 
clearness  the  character  of  wave-motion,  both 
in  ether  and  in  air.  And,  with  this  view.  I 
bring  bcfcre  you  an  experiment  wherein  the 
air-particles  are  represented  by  small  spots  of 
light.  They  are  parts  of  a  spiral,  drawn  upon 
a  circle  of  blackened  glass,  and,  when  the  cir- 
cle rotates,  the  spots  move  in  successive  pulses 
over  the  screen.  You  have  here  clearly  set 
before  you  how  the  pulses  travel  incessantly 
forward,  while  the  particles  that  compose 
them  perform  oscillations  to  and  fro.  This 
is  the  picture  of  a  sound-wave,  in  which  the 
vibrations  are  longitudinal,  By  another  glass 
wheel,  we  produce  an  image  of  a  trans- 
verse wave,  and  here  we  observe  the  waves 
travelling  in  succession  over  the  screen,  while 
each  individual  spot  of  light  performs  an  ex- 
cursion to  and  fro  across  the  line  of  propaga- 
tion. 

Notice  what  follows  when  the  glass  wheel 
is  turned  very  quickly.  Objectively  consid- 
ered, the  transverse  waves  propagate  them- 
selves as  before,  but  subjectively  the  effect  is 
totally  changed.  Because  of  the  retention  of 
impressions  upon  the  retina,  the  spots  of  light 
simply  describe  a  series  of  parallel  luminous 
lines  upon  the  screen,  the  length  of  these 
lines  marking  the  amplitude  of  the  vibration. 
The  impression  of  wave-motion  has  totally 
disappeared. 

The  most  familiar  illustration  of  the  inter- 
ference of  sound-waves  is  furnished  by  the 
beats  produced  by  two  musical  sounds  slightly 
out  of  unison.  These  two  tuning-forks  are 
now  in  perfect  unison,  and  when  they  are 
agitated  together  the  two  sounds  flow  without 
roughness,  as  if  they  were  but  one.  But,  by 
attaching  to  one  of  the  forks  a  two-cent  piece, 
we  cause  it  to  vibrate  a  little  more  slowly 
than  its  neighbor.  Suppose  that  one  of  them 
performs  101  vibrations  in  the  time  required 
by  the  other  to  perform  100,  and  suppose 
that  at  starting  the  condensations  and  rare- 
factions of  both  forks  coincide.  At  the  loist 
vibration  of  the  quickest  fork  they  will  again 
coincide,  the  quicker  fork  at  this  point  hav- 
ing gained  one  whole  vibration,  or  one  whole 
wave  upon  the  other.  But  a  little  reflection 
will  make  it  clear  that,  at  the  5oth  vibration, 
the  two  forks  are  in  opposition;  here  the  one 
tends  to  produce  a  condensation  where  the 
other  tends  to  produce  a  rarefaction;  by  the 
united  action  of  the  two  forks,  therefore,  the 
sound  is  quenched,  and  we  have  a  pause  of 
silence.  This  occurs  where  one  fork  has 
gained  half  a  tuave-/cngl/i  upon  the  other. 
At  the  loist  vibration  we  have  again  coinci 
dence,  and,  therefore,  augmented  sound;  at 
the  isoth  vibration  we  have  again  a  quench- 
ing of  the  sound.  Here  the  one  fork  is  three 
half-waves  in  advance  of  the  other  In  gen- 
eral terms,  the  waves  conspire  when  the  one 
series  is  an  even  number  of  half-wave  lengths, 


SIX  LECTURES  ON  LIGHT. 


13 


and  they  are  destroyed  when  the  one  series  is  band  of  light  gradually  shortening  as  th 

an  odd  number  of  half-wave  lengths  in  ad-  tion  subsides,  until,  when  the  motion  ceases, 

vance  of   the  other.     With  two  forks  so  cir-  we  hare  our  luminous  disk  restored.  Weight- 

cumstanced,    we    obtain   those    intermittent  ing  one  of  the  forks  as  we  did  before,  with  a 


shocks  of   sound  separated  by  pauses  of  si- 
lence, to  which  we  give  the  name  of  beats. 

I  new  wish  to  show  you  what  may  be 
called  the  optical  expression  of  those  beats. 
Attached  to  a  large  tuning-fork,  F  (Fig.  2), 
is  a  small  mirror,  which  shares  the  vibrations 
of  the  fork,  and  on  to  the  mirror  is  thrown  a 
thin  beam  of  light,  which  shares  the  vibra- 
tions of  the  mirror.  The  beam  reflected 
from  the  fork  is  received  upon  a  piece  of 
looking-glass,  and  thrown  back  upon  the 
screen,  where  it  stamps  itself  as  a  small  lu- 
minous disk.  The  agitation  of  the  fork  by  a 


two-cent  piece,  sometimes  the  fork;  conspire, 
and  then  you  have  the  band  of  light  drawn 
out  to  its  maximum  length  ;  sometimes  the? 
oppose  each  other,  and  then  you  have  tha 
band  of  light  diminished  to  a  circle.  Thus, 
the  beats  which  address  the  ear  express  them- 
selves optically  as  the  alternate  elongation  and 
shortening  of  the  band  of  light.  If  I  move 
the  mirror  of  this  second  fork,  you  have  a 
sinuous  line,  as  before  ;  but  the  sinuosities 
are  sometimes  deep,  and  sometimes  they  al- 
most disappear,  as  in  Fig.  3,  thus  expressing 
the  alternate  increase  and  diminution  of  the 


violin-bow  converts  that  disk  into  a  band  of    sound,  the  intensity  of  which  is  expressed  by 
light,  and  if  yoi  simp:y  move  your  heads  to  j  the  depth  of  the  sinuosities.     To  Lissajous  ws 


and  fro  you  cause  the  image  of  the  band  to 
sweep  over  the  retina,  drawing  it  out  to  a  sin- 
uous line,  thus  proving  the  periodic  character 
of  the  motion  which  produces  it.  By  a  sweep 
of  the  looking-glass,  we  can  also  cover  the 
screen  from  side  to  side  by  a  luminous  scroll, 
m  n,  Fig.  2,  the  depth  of  the  sinuosities  indi- 
cating the  amplitude  of  the  vibration. 


Instead  of  receiving  the  beam  reflected  from 
the  fork  on  a  piece  of  looking-glass,  we  now 
receive  it  upon  a  second  mirror  attached  t:>  a 
second  fork,  and  cast  by  it  upon  the  screer. 
Both  forks  now  act  in  combination  upon  the 
beam.  The  disk  is  drawn  out,  as  before,  the 


ou  e  this  mode  of  illustration. 


The  pitch  of  a  sound  is  wholl  /  ^e 
by  the  rapidity  of  the  vibration,  ay  tLie  ,nten- 
sity'is  by  the  amplitude.     The  us*:  of   pitch 
with  the  rapidity  of  the  impulses  may  be  illus- 
trated by  the  syren,  which  consists  of  a  per- 
forated disk    rotating    over   a  cylinder   into 
which  air  is  forced,  and  the  end  of  which  is 
also   perforated.     When   the  perforations  o£ 
the  disk  coincide  with  those  of  the  cylinder,  a 
puff  escapes ;  and,  when  the  puffs  succeed 
each   other   with   sufficient   rapidity,  the  im- 
|  pressions  upon  the  auditory  nerve  link  them- 
selves together  to  a  continuous  musical  note. 
|  The  more  r"apid  the  rotation  of  the  disk  the 
j  quicker  is  the  succession  of  the  impulses,  and 
'  the  higher  the  pitch  of  the  note.     Indeed,  by 


SIX  LECTURES  ON  LIGHT. 


ireans  cf  the  syren  the  number  of  vibrations 
due  to  any  musical  no'e,  whether  it  be  that  of 
an  instrument,  of  the  human  voice,  or  of  a 
flying  insect,  may  be  accurately  determined. 


In  front  of  our  lamp  now  stands  a  very 
homely  instrument,  S,  Fi<?-.  4,  of  this  charac- 
ter. The  perforated  disk  is  turned  by  a 
wheel  and  band,  and,  when  the  two  sets  of 
perforations  coincide,  a  series  of  spots  of 
light,  sharply  denned  by  the  lens  L,  ranged 
on  the  circumference  of  a  circle,  is  seen  upon 
the  screen.  On  slowly  turning  the  disk,  a 
flicker  is  produced  by  the  alternate  stoppage 
and  transmission  of  the  light.  At  the  same 
time  air  is  urged  into  the  syren,  and  you  hear 
a  fluttering  sound  corresponding  to  the  flick- 
ering light.  But,  by  augmenting  the  rapid- 
ity of  rotation,  the  light,  though  intercepted 
as  before,  appears  perfectly  steady,  through 
the  persistence  of  impressions  upon  the 
retina  ;  and,  about  the  time  when  the  optical 
impression  becomes  continuous,  the  auditory 
impression  becomes  equally  so ;  the  puffs 
from  the  syren  linking  themselves  then  to- 
gether to  a  continuous  musical  note,  which 
rises  in  pitch  with  the  rapidity  of  the  rota- 
lion.  A  movement  of  the  head  causes  the 
image  of  the  spots  to  sweep  over  the  retina, 
producing  beaded  lines  :  the  same  effect  is 
produced  upon  our  screen  by  the  sweep  of  a 
looking  glass  which  has  received  the  thin 
beams  from  the  syren. 

In  the  undulatory  theory,  what  pitch  is  to 
the  ear,  cclor  is  to  the  eye.  Though  never 
seen,  the  lengths  of  the  waves  of  light  have 
been  determined.  Their  existence  is  proved 
.by  their  effects,  and  from  their  effects  also 
their  lengths  may  be  accurately  deduced. 
This  may,  moreover,  be  done  in  many  ways, 
•ami,  when  the  different  determinations  are 


compared,  the  strictest  harmony  is  found  to 
exist  between  them.  The  shortest  waves-  of 
the  visible  spectrum  are  those  of  the  extreme 
violet  ;  the  longest,  those  of  the  extreme 
red  ;  while  the  other  colors  are  of  intermedi- 
ate pitch  or  wave-length.  The  length  of  a 
wave  of  the  extreme  red  is  such  that  it  would 
require  36,918  of  them  placed  end  to  end  to 
cover  one  inch,  while  64,631  of  the  extreme 
violet  waves  would  be  required  to  span  the 
same  distance. 

Now,  the  velocity  of  light,  in  round  num- 
bers, is  190,000  miles  per  second.  Reducing 
this  to  inches,  and  multiplying  the  number 
ihus  found  by  36,918,  we  obtain  the  number 
of  waves  of  the  extreme  red  in  190,000  miles. 
All  these  "waves  enter  the  eye,  and  hit  the 
retina  fit  the  back  of  the  eye  in  one  second. 
The  number  of  shocks  per  second  necessary 
to  the  production  of  the  impression  of  red  is, 
therefore,  four  hundred  and  fifty-one  millions 
of  millions.  In  a  similar  manner,  it  may  be 
found  that  the  number  of  shocks  correspond- 
ing to  the  impression  of  violet  is  seven  hun- 
dred and  eighty-nine  millions  of  millions. 
All  space  is  rilled  with  matter  oscillating  at 
such  rates.  From  every  star  waves  of  these 
dimensions  move  with  the  velocity  of  light 
like  spherical  shells  outwards.  And  in  the 
ether,  just  as  in  the  water,  the  motion  of 
cveiy  particle  ij  the  algebraic  sum  cf  all  the 
separate  motions  imparted  to  it.  Still,  one 
motion  does  not  blot  the  other  out ;  or,  if 
extinction  occur  at  one  point,  it  is  atoned  for 
at  some  other  point.  Every  star  declares  by 
its  light  its  undamaged  individuality,  as  if  it 
alone  had  sent  its  thrills  through  space. 

The  principle  of  interference  applies  to  the 
waves  of  light  as  it  does  to  the  waves  of 
water  and  the  waves  of  sound.  And  the 
condi  ions  of  interference  are  the  same  in  all 
three.  If  two  series  of  light-waves  of  the 
same  length  start  at  the  same  moment  from 
a  common  origin,  crest  coincides  with  crest, 
sinus  with  sinus,  and  the  two  systems  blend 
together  to  a  single  system  of  double  ampli- 
tude. If  both  series  start  at  the  same  mo- 
ment, one  of  them  being,  at  starting,  a  whole 
wave-length  in  advance  of  the  other,  they 
also  add  themselves  together,  and  we  have 
an  augmented  luminous  effect.  Just  as  in 
the  case  of  sound,  the  same  occurs  when  the 
one  system  of  waves  is  any  even  number  of 
semi-undulations  in  advance  of  the  other. 
But  if  the  one  system  be  half  a  wave-length, 
or  any  odd  number  of  half  wave-lengths  in 
advance,  then  the  crests  of  the  one  fail  upon 
the  sinuses  of  the  other  ;  the  one  system,  in 
fact,  tends  to  lift  the  particles  of  ether  at  the 
precise  places  where  the  other  tends  to  depress 
them  ;  hence,  through  their  joint  action  the 
ether  remains  perfectly  stiil  This  stillness 
of  the  ether  is  what  we  call  darkness,  which 
corresponds,  as  already  stated,  with  a  dead 
level  in  the  case  of  water. 

It  was  said  in  our  first  lecture,  with  refer- 
ence to  the  colors  produced  by  absorption, 


SIX  LECTURES  ON  LIGHT. 


that  the  function  of  natural  bodies  is  selec- 
tive, not  creative  ;  that  they  extinguish  cer- 
tain constituents  of  the  \vhite  solar  light,  and 
appear  in  the  colors  of  the  unextinguished 
iight.  It  must  atonce  flash  upon  your  minds 
that,  inasmuch  as  we  have  in  interference  an 
agency  by  which  light  may  be  self-extin- 
quished,  we  may  have  in  it  the  conditions 
for  the  production  of  color.  But  this  would 
imply  that  certain  constituents  are  quenched 
by  interference,  while  others  are  permitted  to 
remain.  This  is  the  fact  ;  and  it  is  entirely 
due  to  the  difference  in  the  lengths  of  the 
waves  of  light. 

The  subject  is  most  easily  illustrated  by  the 
class  of  phenomena  which  (irst  suggested  the 
undulatory  theory  to  the  mind  of  Hooke. 
These  are  the  colors  of  thin  films  of  all  kinds, 
which  are  known  as  the  colors  of  thin  plates. 
In  this  relation  no  object  in  the  world  pos- 
sesses a  deeper  scientific  interest  than  a  com- 
mon soap-bubble.  And  here  let  me  say 
emerges  one  of  the  difficulties  which  the  stu- 
dent of  pure  science  encounters  in  the  pres- 
ence of  "practical"  communities  like  those 
of  America  and  England  ;  it  is  not  to  be  ex- 
pected that  such  communities  can  entertain 
any  profound  sympathy  with  labors  which 
seem  so  far  removed  from  the  domain  of 
practice  as  many  of  the  labors  of  the  man  of 
science  are.  Imagine  Dr.  Draper  spending 
his  days  in  blowing  soap-bubbles  and  in 
studying  their  colors  !  Would  you  show  him 
the  necessary  patience,  or  grant  him  the  nec- 
essary support  ?  And  yet,  be  it  remembered, 
it  was  thus  that  Newton  spe::t  a  large  portion 
of  his  time  ;  and  that  on  such  experiments 
has  been  founded  a  theory,  the  issues  of 
which  are  incalculable.  I  see  no  other  way 
for  you  laymen  than  to  trust  the  scientific  man 
with  the  choice  of  his  inquiries  ;  he  stands 
before  the  tribunal  of  his  peers,  and  by  their 
verdict  on  his  labors  you  ought  to  abide. 

Whence,  then,  are  derived  the  colors  of  the 
soap-bubble  ?  Imagine  abeam  of  white  l:ght 
impinging  on  the  bubble.  When  it  reaches 
the  first  surface  of  the  film,  a  known  fraction 
of  the  light  is  reflected  back.  But  a  large 
portion  of  the  beam  enters  the  film,  reaches 
its  second  surface,  and  is  again  in  part  re- 
flected. The  waves  from  the  second  surface 
thus  turn  back  and  hotly  pursue  the  waves 
from  the  first  surface.  And,  if  the  thickness 
of  the  film  be  such  as  to  cause  the  necessary 
retardation,  the  two  systems  of  waves  inter- 
fere with  each  other,  producing  augmented 
or  diminished  light,  as  the  case  may  be.  But, 
inasmuch  as  the  waves  of  light  are  of  different 
lengths,  it  is  plain  that,  to  produce  self-ex- 
tinction in  the  case  of  the  longer  waves,  a 
greater  thickness  of  film  is  necessary  than  in 
the  case  of  the  shorter  ones.  Different  colors, 
therefore,  appear  at  different  thicknesses  of 
the  film. 

Take  with  you  a  little  bottle  of  spirit  of 
turpentine,  and  pour  it  into  one  of  the  ponds 
in  the  Central  Park.  You  will  then  see  the 


flashing  of  those  colors  over  the  surface  of 
the  wa^er.  On  a  small  scale  we  produce  them 
thus  :  A  common  tea-tray  is  filled  with  water, 
beneath  the  surface  of  which  dips  the  end  of 
a  pipette.  A  beam  of  light  falls  upon  the 
water,  and  is  reflected  by  it  to  the  screen. 
Spirit  of  turpentine  is  poured  into  the  pipette; 
it  descends,  issues  from  the  end  in  minute 
drops,  which  lise  in  -  uccession  to  the  surface. 
i  On  reaching  it,  each  drop  spreads  suddenly 
j  out  as  a  film,  and  glowing  colors  immediately 
j  flash  forth  upon  the  screen.  The  colors 
change  as  the  thickness  of  the  film  changes 
by  evaporation.  They  are  also  arranged  in 
zones  in  consequence  of  the  gradual  diminu- 
tion of  thickness  from  the  centre  outwards. 

Any  film  whatever  will  produce  these  colors. 
The  film  of  air  between  two  plates  of  window- 
glass,  squeezed  together,  exhibits  rich  fringes 
of  color.  Nor  is  even  air  necessary  ;  the 
mere  rupture  of  optical  continuity  suffices. 
Smite  with  an  axe  the  black,  transparent  ice — 
black,  because  it  is  transparent  and  of  great 
depth — under  the  moraine  of  a  glacier  ;  you 
readily  produce  in  the  interior  flaws  which  no 
air  can  reach,  and  from  these  flaws  the  colors 
of  thin  plates  sometimes  break  like  fire.  The 
colors  are  commonly  seen  in  flawed  crystals  ; 
they  are  also  formed  by  the  film  of  oxide 
which  collects  upon  molten  lead.  It  is  the 
colors  of  thin  plates  that  guide  the  tempering 
of  steel.  But  the  origin  of  most  historic  in- 
terest is,  ns  already  stated,  the  soap-bubble. 
With  one  of  those  mixtures  employed  by  the 
eminent  blind  philosopher  Plateau  in  his  re- 
searches on  the  cohesion  figures  of  thin  films, 
\\e  obtain  in  still  airabv.bblc  twelve  or  fifteen 
inches  in  diameter.  You -may  look  at  the 
bubble  itself,  or  you  may  look  at  its  projec- 
tion upon  the  screen,  rich  colors  arranged  in 
zones  are,  in  both  cases,  exhibited.  Render- 
ing the  beam  parallel,  and  permitting  it  to 
impinge  upon  the  sides,  bottom,  and  top  of 
the  bubble,  gorgeous  fans  of  color  overspread 
the  screen,  which  rotate  as  the  beam  is  carried 
round  the  circumference  of  the  bubble.  By 
this  experiment  the  internal  motions  of  the 
film  are  also  strikingly  displayed. 

Newton  sought  to  measure  the  thickness  of 
the  bubble  corresponding  to  each  of  these 
colors  ;  in  fact,  he  sought  to  determine  gen- 
erally the  relation  of  color  to  thickness.  His 
first  care  was  to  obtain  a  film  of  variable  and 
calculable  depth.  On  a  plano-convex  glass 
lens  of  very  feeble  curvature  he  laid  a  plate  of 
glass  with  a  plane  surface,  thus  obtaining  a 
him  of  air  of  gradually  increasing  depth  from 
the  point  of  contact  outwards.  On  looking  at 
the  film  in  monochromatic  light  he  saw  su:- 
roundingthe  place  of  contact  a  series  of  bright 
rings  separated  from  each  other  by  dark  ones, 
and  becoming  more  closely  packed  together  as 
the  distance  from  the  point  of  conta  t  aug- 
mented. When  he  employed  red  light,  his 
rings  had  certain  diameters  ;  when  he  em- 
ployed blue  light,  the  diameters  were  less. 
Causing  his  glasses  to  pass  through  the  spev/ 


SIX  LECTURES  ON  LIGHT- 


trurn  from  red  to  blue,  the  rings  contracted  ; 
when  the  passage  was  from  blue  to  red,  the 
rings  expanded.  When  white  light  fell  upon 
the  glasses,  inasmuch  as  the  colors  were  not 
superposed,  a  series  of  iris-colored  circles  were 
obtained.  They  became  paler  as  the  film  be- 
came thicker,  until  finally  ihe  colors  became 
so  intimately  reblended  as  to  produce  white 
light.  A  magnified  image  of  Newton's  rings 
is  now  before  you,  and,  by  employing  in  suc- 
cession red,  blue,  and  white  light,  we  obtain 
all  the  effects  observed  by  Newton. 

He  compared  the  tints  thus  obtained  v  ith 
the  tints  of  the  sda--bubble,  and  he  calcu- 
lated the  corresponding  thickness.  How  he 
did  this  may  be  thus  made  plain  to  you  : 
Suppose  the' water  of  the  ocean  to  be  abso- 
lutely smooth  ;  it  would  then  accurately  repre- 
sent the  earth's  curved  surface.  Let  a  per- 
fectly horizontal  plane  touch  the  surface  at 
any  point.  Knowing  the  earth's  diameter, 
any  engineer  or  mathematician  in  this  room 
could  tell  you  how  far  the  sea's  surface  will 
lie  below  this  plane,  at  the  distance  of  a  yard, 
ten  yards,  a  hundred  yards,  or  a  thousand 
yards  from  the  point  of  contact  of  the  plane 
and  the  sea.  It  is  common,  indeed,  in  lev- 
elling operations,  to  a'. low  for  the  curvature 
of  the  earth.  Newton's  calculation  was  pre- 
cisely similar.  His  plane  glass  was  a  tan- 
gent to  his  curved  cne  From  its  refractive 
index  and  focal  distance  he  determined  the 
diameter  of  the  sphere  of  which  his  curved 
glass  formed  a  segment,  he  measured  the 
distances  of  his  rings  from  the  place  of  con- 
tact, and  he  calculated  the  depth  between  the 
tangent  plane  and  the  curved  surface,  exactly 
as  the  engineer  would  calculate  the  distance 
between  his  tangent  plane  and  the  surface  of 
the  sea.  The  wonder  is,  that,  where  such 
infinitesimal  distances  arc  involved,  Newton, 
with  the  means  at  his  disposal,  could  have 
worked  with  such  marvellous  exactitude. 

To  account  for  these  rings  was  the  great- 
est difficulty  that  Newton  ever  encoun- 
tered. He  quite  appreciated  the  difficulty. 
Over  his  eagle-eye  there  was  no  film — no 
vagueness  in  his  conceptions.  At  the  very 
outset  his  theory  was  confronted  by  the  ques- 
tion, "Why,  when  a  beam  of  light  is  incident 
on  a  transparent  body,  are  some  of  the  light- 
particles  reflected  and  some  transmitted  ?  Is 
it  that  there  are  two  kinds  of  particles,  the 
one  specially  fitted  for  transmission  and  the 
other  for  reflection?  This  cannot  be  the 
reason  ;  for,  if  we  allow  a  beam  of  light 
which  has  been  reflected  from  one  piece  of 
glass  to  fall  upon  another,  it,  as  a  general 
rule,  is  also  divided  into  a  reflected  and  a 
transmitted  portion.  Thus  the  particles  once 
reflected  are  not  always  reflected,  nor  are  the 
particles  once  transmitted  always  transmitted. 
Newton  saw  all  this  ;  he  knew  he  had  to  ex 
plain  why  it  is  that  the  self-same  particle  is 
at  one  moment  reflected  and  at  the  next  mo- 
ment transmitted.  It  could  only  be  through 
change  in  the  condition  of  the  particle 


itself.  The  self-same  particle,  he  affirmed, 
was  affected  by  "  fits"  of  easy  transmission 
and  reflection. 

If  you  are  willing  to  follow  me  while  I  un- 
ravel this  theory  of  fits,  the  most  subtle,  per- 
haps, that  ever  entered  the  human  mind,  the 
intellectual  discipline  will  repay  you  for  the 
necessary  effort  of  attention.     Newton   was 
chary  of  stating  what  he  considered  to  be  the 
cause  of  the  fits,  but  there  cannot  be  a  doubt 
that  his  mind  rested  on  a  mechanical  cause. 
Nor  can   there  be   a  doubt  that,    as   in   all 
attempts  at  theorizing,  he  was  compelled  to 
fall  back  upon  experience  for  the  materials  of 
his  theory.     His  course  of  observation  anqf 
of   thought   may   have   been   this  :    From   a 
magnet  he  might  obtain  the  notion  of   at- 
tracted    and    repelled    poles.      What    more 
natural  than  that  he  should  endow  his  light- 
particles   with    such   poles  ?      Turning  their 
attracted   poles   towards   a  transparent   sub- 
stance, the  particles  would  be  sucked  in  and 
transmitted ;    turning  their    repelled    poles, 
they  would   be   driven    away    or    reflected. 
Thus,  by  the  ascription  of  poles,  the  trans- 
mission and  reflection  of  the  self-same  parti- 
cle at  different  times  might  be  accounted  for. 
Regard  these  rings  of  Newton  as  seen  in 
pure  red   light :  they   are   alternately  bright 
and  dark.     The  film   of  air  corresponding  to 
the  outermost  of  them  is  not  thicker  than  an 
ordinary  soap-bubble,  and  it  becomes  thinner 
on  approaching  the  centre  ;  still  Newton,  as 
I  have   said,  measured  the   thickness  corre- 
sponding to  every  ring  and  showed  the  differ- 
ence of   thickness   between   ring   and    ring. 
Now,  mark  the  result.     For  the  sake  of  con- 
venience, let  us  call  the  thickness  of  the  film 
of  air  corresponding  to  the  first  dark  ring  d, 
then  Newton  found  the  distance  correspond- 
ing to  the   second  dark  ring'  2  d;  the  thick 
ness  corresponding  to  the  third  dark  ring:  3  d; 
the  thickness  corresponding  to  the  tenth  dark 
ring  10  d,  and  so  on.     Surely  there  must  be 
some  hidden  meaning  in  this  little  distanced, 
which  turns  up  so  constantly  ?     One  can  im- 
agine the  intense  interest  with  which  Newton 
pondered  its  meaning.     Observe  the  probably 
outcome  of  his   thought.     He  had  endowed 
his  light-particles  with   poles,  but   now  he  is 
forced  to  introduce  the   no'. ion  of  periodic  re- 
currence.    How  was   this   to  be  done  ?     By 
supposing    the  light -particles  animated,  not 
only    with     a    motion  of     translation,    but 
also  with  a   motion  of   rotation.     Newton's 
astronomical  knowledge  would  render  all  such 
conceptions  familiar  to  him.     The  earth  has 
such  a  motion.     In  the  time  occupied  in  pass- 
ing over  a  million  and  a  half  of  miles  of  its 
orbit — that    is     in    twenty  four    hours — our 
planet  performs  a  complete  rotation,  and,  in 
the  time  required  to  pass  over  the  distance  d, 
Newton's  light-particle  must  be  supposed  to 
perform    a    complete    rotation.     True,     the 
light-particle   is  smaller  than  the  planet    and 
the  distance  d,  instead  of  being  a  million  and 
a  half  of  miles,  is  a   little  over  the  ninety- 


SIX  LECTURES  ON  LIGHT/ 


ir 


thousandth  of  an  inch.  But  the  two  con- 
ceptions are,  in  point  of  intellectual  quality, 
identical. 

N  Imagine,  then,  a  particle  entering  the  film 
of  air  where  it  possesses  this  precise  thick- 
ness. To  enter  the  film,  its  attracted  end 
must  be  presented.  Within  the  film  it  is  able 
to  turn  once  completely  round  ;  at  the  other 
side  of  the  film  its  attracted  pole  will  be  again 
presented  ;  it  will,  therefore,  enter  the  glass 
at  the  opposite  side  of  the  film  and  be  lost  to 
the  eye.  All  round  the  place  of  contact, 
wherever  the  film  possesses  this  precise  thick- 
ness, the  light  will  equally  disappear — we 
shall  have  a  ring  of  darkness. 

And  now  observe  how  well  this  conception 
falls  in  with  the  law  of  proportionality  dis- 
covered by  Newton.  When  the  thickness  of 
the  film  is  2  </,  the  particle  has  time  to  per- 
form two  complete  somersaults  within  the 
film  ;  when  the  thickness  is  3  d,  three  com- 
plete somersaults  ;  when  10  d,  ten  complete 
somersaults  are  performed.  It  is  manifest 
that  in  each  of  these  cases,  on  arriving  at  the 
second  surface  of  the  film,  the  attracted  pole 
of  the  particle  will  be  presented.  It  will, 
therefore,  be  transmitted,  and,  because  no 
light  is  sent  to  the  eye,  we  shall  have  a  ring 
of  darkness  at  each  of  these  placss. 

The  bright  rings  follow  immediately  from 
the  same  conception.  They  occur  between 
the  dark  rings,  the  thicknesses  to  which  they 
correspond  being  also  intermediate  between 
those  of  the  dark  ones.  Take  the  case  of  the 
first  bright  ring.  The  thickness  of  the  film 
is  l/t  d;  in  this  interval  the  rotating  particle 
can  perform  only  half  a  rotation.  When, 
therefore,  it  reaches  t.'.e  second  surface  of  the 
film,  its  repelled  pole  is  presented  ;  it  is, 
therefore,  driven  back  and  reaches  the  eye. 
At  all  distances  round  the  centre  correspond- 
r~.g  to  this  thickness  the  same  effect  is  pro- 
duced, and  the  consequence  is  a  ring  of 
brightness.  The  other  bright  rings  are  sim- 
ilarly accounted  for.  At  the  second  one, 
where  the  thickness  is  i^  d,  a  rotation  and 
a  half  is  performed  ;  at  the  third,  two  rota- 
tions and  a  half  ;  and  at  each  of  these  places 
the  particles  present  their  repelled  poles  to 
the  lower  surface  of  the  film.  They  are  there- 
fore sent  back  to  the  eye,  producing  the  im- 
pression of  brightness.  Here,  then,  we  havo 
unravelled  the  most  subtle  application  that 
Newton  ever  made  of  the  Emission  Theory. 

It  has  been  stated  in  the  early  part  of  this 
lecture,  that  the  Emission  Theory  assigned  a 
greater  velocity  to  light  in  glass  and  water, 
than  in  air  or  stellar  space.  Here  it  was  at 
direct  issue  with  the  theory  of  undulation, 
v/hich  makes  the  velocity  in  air  or  stellar 
space  less  than  in  glass  or  water.  By  an  ex- 
periment proposed  by  Arago,  and  executed 
with  consummate  skill  by  Foucault  and 
Fizeau,  this  question  was  b  -ought  to  a  crucial 
test,  and  decided  in  favor  of  the  theory  of 
i.mdulation.  In  the  present  instance  also  the 
two  theories  are  at  variance.  Newton  as- 


sumed that  the  action  which  produces  the  al- 
ternate bright  and  dark  rings  took  place  at  a 
single  surface  ;  that  is,  the  second  surface  of 
the  film.  The  undulatory  theory  affirms 
that  the  rings  are  caused  by  the  interference 
of  waves  reflected  from  both  surfaces.  This 
also  has  been  demonstrated  by  experiment. 
By  proper  devices  we  may  abolish  reflection 
from  one  of  the  surfaces  of  the  film,  ancj 
when  this  is  done  the  rings  vanish  altogether. 

Rings  of  feeble  intensity  are  also  formed  by 
transmitted  light.  These  are  referred  by  the, 
undulatcry  theory  to  the  interference  of 
waves  which  have  passed  directly  through  the 
film,  with  others  which  have  suffered  two  re- 
flectionswithin  the  film.  They  are  thus  com- 
pletely accounted  for. 

Newton,  by  the  foregoing  exceedingly 
subtle  assumption,  vaulted  over  the  difficulty 
presented  by  the  colors  of  thin  plates.  And, 
as  further  difficulties  in  process  of  time  thick- 
ened round  the  theory,  his  disciples  tried  to 
sustain  it  with  an  ingenuity  worthy  of  their 
master.  The  new  difficulties  were  not  an- 
ticipated by  the  theory,  but  were  met  by  new 
assumptions,  until  at  length  the  Emission 
Theory  became  what  a  distinguished  writer 
calls  a  "  mob  of  hypotheses."  In  the  pres- 
ence of  the  phenomena  of  interference,  the 
theory  finally  broke  down,  while  the  whole  of 
these  phenomena  lie,  as  it  were,  latent  in  the 
theory  of  undulation.  Newton's  "  fits,"  for 
example,  are  immediately  translatable  into 
the  lengths  of  the  ether-waves.  We  have 
the  observed  periodic  recurrence  as  the  thick- 
ness varies  so  as  to  produce  a  retardation  of 
an  odd  or  even  number  of  semi-undulations.* 

Numerous  other  colors  are  due  to  interfer- 
ence. Fine  scratches  drawn  upon  glass  ot 
polished  metal  reflect  the  waves  of  light  from 
their  sides;  and  some,  being  reflected  from 
opposite  sides  of  the  same  furrow,  interfere 
with  and  quench  each  other.  But  the  ob- 
liquity of  reflection  which  extinguishes  the 
shorter  waves  does  not  extinguish  the  longer 
ones,  hence  the  phenomena  of  color.  These 
are  called  the  colors  of  striated  surfaces. 
They  are  well  illustrated  by  mother-of-pearl. 
This  shell  is  composed  of  exceedingly  thin 
layers,  which,  when  cut  across  by  the  polish- 
ing of  the  shell,  expose  their  edges  and  fur- 
nish the  necessary  small  and  regular  grooves. 
The  most  conclusive  proof  that  the  colors  ar«- 
due  to  the  mechanical  state  of  tr.e  surface  i  -. 
to  be  found  in  the  fact,  established  by  Brew 
ter,  that,  by  stamping;  the  shell  carefully 

*  In  the  explanation  of  Newton's  rings,  something 
besides  thickness  is  to  be  taken  into  account.  In  the 
case  of  the  first  surface  of  the  film  of  air,  the  waves 
pass  from  a  denser  to  a  r?rer  medium,  while  in  the 
case  of  the  second  surface,  the  waves  pass  from  a 
rarer  to  a  denser  medium.  This  difference  at  the 
two  reflecting  surfaces  can  be  proved  to  be  equivalent 
to  the  addition  of  Jialf  a  wave-length  to  the  thick- 
ness of  the  film.  To  the  absolute  thickness,  as  de- 
ermined  by  Newton,  half  a  wave-length  is  in  each 
:ase  to  be  added.  When  this  is  done,  the  dark  and 
Bright  rings  follow  each  other  in  exact  accordance 
with  the  law  of  interference  already  enunciated. 


SIX  LECTURES  ON  LIGHT. 


upon  black  sealing-wax,  we  transfer  the 
grooves,  and  produce  upon  the  wax  the  colors 
of  *nother-of-pearl. 


LECTURE  III. 

Relation  of  Theories  to  Experience :  Origin  of  the 
Notion  of  the  Attraction  of  Gravitation  :  Notion 
of  Polarity,  how  generated :  Atomic  Polarity : 
Structural  Arrangements  due  to  Polarity  :  Archi- 
tecture of  Crystals  considered  as  an 'Introduction  to 
their  Action  upon  Light:  Notion  of  Atomic  Po- 
larity applied  to  Crystalline  Structure :  Experi- 
mental Illustrations:  Crystallization  of  Water: 
Expansion  by  Heat  and  by  Cold  :  Deportment  of 
Water  considered  and  explained:  Molecular  Ac- 
tion illustrated  by  a  Model:  Force  of  Solidifica- 
tion :  Bearings  of  Crystallization  on  Optical  Phe- 
nomena :  Refraction :  Double  Refraction  :  Po- 
larization:  Action  of  Tourmaline:  Character  of 
the  Beams  emergent  from  Iceland  Spar  :  Polariza- 
tion by  ordinary  Refraction  and  Reflection  :  De- 
polarization. 

IN  our  last  lecture  we  sought  to  familiarize 
Our  minds  with  the  characteristics  of  wave- 
motion.  We  drew  a  clear  distinction  between 
the  motio  i  of  the  wave  itself  and  the  motion 
of  its  constituent  particles.  Passing  through 
water-waves  and  air-waves,  we  prepared  our 
mi  ds  for  the  conception  of  light-waves  prop- 
agated through  the  luminiferous  ether.  The 
analogy  of  sound  will  fix  the  whole  mechan- 
ism in  your  minds.  Here  we  have  a  vibrat- 
ing body  which  originates  the  wave  motion, 
we  have,  in  the  air,  a  vehicle  which  conveys 
it,  and  we  have  the  auditory  nerve  which  re- 
ceives the  impressions  of  the  sonorous  waves. 
In  the  case  of  light  we  have  in  the  vibrating 
atoms  of  the  luminous  body  the  originators  of 
the  wave-motion,  we  have  in  the  ether  its 
vehicle,  while  the  optic  nerve  receives  the  im- 
pression of  the  luminiferous  waves.  We 
learned,  also,  that  color  is  the  analogue  of 
pitch,  that  the  rapidity  of  atomic  vibration 
augmented,  and  the  length  of  the  ether-waves 
decreased,  in  passing  from  the  red  to  the  blue 
end  of  the  spectrum.  The  fruitful  principle 
of  interference  we  also  found  applicable  to 
the  phenomena  of  light  ;  and  we  learned  that, 
in  consequence  of  the  different  lengths  of  the 
ether- waves,  they  were  extinguished  by  dif- 
ferent thicknesses  of  a  transparent  film,  the 
particular  thickness  which  quenched  one  color 
glowing,  therefore,  with  the  complementary 
one.  Thus  the  colors  of  thin  plates  were  ac- 
counted for. 

But  one  of  the  objects  of  our  last  lecture, 
and  that  not  the  least  important,  was  to  illus- 
trate the  manner  in  which  scientific  theories 
are  iormed.  They,  in  the  first  place,  take 
their  rise  in  the  desire  of  the  mind  to  pene- 
trate to  the  sources  of  phenomena.  This  de- 
sire has  long  been  a  part  of  human  nature. 
It  yrompted  Caesar  to  say  that  he  would  ex- 
change his  victories  for  a  rlirnpse  of  the 
sources  of  the  Nile  ;  it  may  be  seen  working 
in  Lucretius  ;  it  impels  Darwin  to  those  dar- 
ing speculations  which  of  late  years  have  so 
agitated  the  public  mind.  We  have  learned  ' 
that  in  framing  theories  the  imagination  does 


not  create,  but  that  it  expands,  diminishes, 
moulds,  and  refines,  as  the  case  may  be, 
mater  als  derived  from  the  world  of  fact  and 
observation. 

This  is  more  evidently  the  case  in  a  theory 
like  that  of  light,  where  the  motions  of  a  sub- 
sensible  medium,  the  ether,  are  presented  to 
the  mind.  But  no  theory  escapes  the  condi- 
tion. Newton  took  care  not  to  encumber 
gravitation  with  unnecessary  physical  concep- 
tions ;  but  we  have  reason  to  kno.v  that  he 
indulged  in  them,  though  he  did  not  connect 
them  with  his  theory.  But  even  the  theory 
as  it  stands  did  not  enter  the  mind  ai'  a  reve- 
lation dissevered  from  the  world  of  experi- 
ence. The  germ  of  the  conception  that  the 
sun  and  planets  are  held  together  by  a  force 
of  attraction  is  to  be  found  in  the  fact  that  a 
magnet  had  been  previously  seen  to  attract 
iron.  The  notion  of  matter  attracting  matter 
came  thus  from  without,  not  from  within.  In 
our  present  lecture  the  magnetic  force  must 
serve  us  •  till  further  ;  but  here  we  must  master 
its  elementary  phenomena. 

The  general  facts  of  magnetism  are  most 
simply  illustrated  by  a  magnetized  bar  of 
steel,  commonly  called  a  bar  magnet.  Placing 
such  a  magnet  uyrfurht  upon  a  table,  and 
bringing  a  magne,  ieedle  near  its  bottom, 
one  end  of  the  rtf"r*.>K  promptly  retreats  from 
the  magnet,  v">  the  other  as  promptly 
approaches.  '  aeedle  is  held  quivering 
there  by  some  ^risible  influence  exerted 
upon  it.  Raising  the  needle  along  the  mag- 
net, but  still  avoiding  contact  the  rapidity 
of  its  oscillations  decreases,  because  the  force 
acting  upon  it  becomes  weaker.  At  the 
centre  the  oscillations  cease.  Above  the 
centre,  the  end  of  the  needle  which  had  been 
previously  drawn  towards  the  magnet  re- 
treats, and  the  opposite  end  approaches.  As 
we  ascend  higher,  the  oscillations  become 
mere  violent,  because  the  force  becomes 
stronger.  At  the  upper  end  of  the  magnet, 


FIG.  5. 

as  at  the  lower,  the  force  reaches  a  maximum, 
t>ut  all  the  lower  half  of  the  magnet,  from 
E  to  S  (Fig.  5),  attracts  one  end  of  the 


SIX  LECTURES  ON  LIGHT. 


19 


needle,  while  all  the  upper  half,  from  E  to 
N,  attracts  the  opposite  end.  This  double- 
ness  of  the  magnetic  force  is  called  polarity, 
and  the  points  near  the  ends  of  the  magnet  in 
which  the  forces  seem  concentrated  are  called 
its  poles. 

What,  then,  will  occur  if  we  break  this 
magnet  in  two  at  the  cent:e  E  >  Will  each 
of  the  separate  halves  act  as  it  did  when  it 
formed  part  of  the  whole  magnet?  No; 
each  half  is  in  itself  a  perfect  magnet,  pos 
sessing  two  poles.  This  may  be  proved  by 
breaking  something  of  less  value  than  the 
magnet — the  steel  of  a  lady's  stays,  for  ex- 
t  xample,  hardened  and  magnetized.  •  It  acts 
like  the  magnet.  When  broken,  each  half 
acts  like  the  whole  ;  and  when  these  parts 
are  again  broken,  we  have  still  the  perfect 
magnet,  possessing,  as  in  the  first  instance, 
two  poles.  Push  your  breaking  to  its  utmost 
limit ;  you  will  be  driven  to  prolong  your 


rection  of  the  needle,  and  no  other.  A  needle 
of  iron  will  answer  as  well  as  the  magnetic 
needle  ;  for  the  need  e  of  iron  is  magnetized 
by  the  magnet,  and  acts  exactly  like  a  needle 
independently  magnetized. 

If  we  place  two  or  more  needles  of  iron 
near  the  magnet,  the  action  becomes  more 
complex,  for  the  the  iron  needles  are  not  only;' 
acted  on  by  the  magnet,  but  they  act  upon 
each  other.  And  if  we  pass  to  smaller  masses 
of  iron — to  iron  filings,  for  example — we  find 
that  they  act  substantially  as  the  needles,  ar- 
ranging themselves  in  definite  forms,  in  obe- 
dience to  the  magnetic  action. 

Placing  a  sheet  of  paper  or  glass  over  this 
bar  magnet  and  showering  iron  filings  upon 
the  paper,  I  notice  a  tendency  of  the  filings 
to  arrange  themselves  in  determinate  lines. 
They  cannot  freely  follow  this  tendency,  for 
they  are  hampered  by  the  friction  against  che 
paper,  They  are  helped  by  tapping  the 


FIG.  6. 

K  is  the  nozzle  of  the  lamp  ;  M  a  plane  mirror,  reflecting:  the  beam  upwards.  At  P,  the  magnets  and 
iron  filings  are  placed  ;  L  is  a  lens  which  forms  an  image  of  the  magnets  and  filings  ;  and  R  is  a  total' 
ly- reflecting  prism  which  casts  the  image,  G,  upon  the  screen. 


vision  beyond  that  limit,  and  to  contemplate 
this  thing  that  we  call  magnetic  polarity  as 
resident  in  t)ie  ^lltimate  particles  of  the  mag- 
net. Each  atom  is  endowed  with  this  polar 
force. 

Like  all  other  forces,  this  force  of  magnet- 
ism is  amenable  to  mechanical  laws  ;  and 
knowing  the  direction  and  magnitude  of  the 
force,  we  can  predict  its  action.  Placing  a 
small  magnetic  needle  near  a  bar  magnet,  it 
takes  up  a  determinate  position.  That  posi- 
tion might  be  deduced  theoretically  from  the 
mutual  action  of  the  poles.  Moving  the 
needle  round  the  magnet,  for  each  point  of 
the  surrounding  space  there  is  a  definite  di- 


paper:  each  tap  releases  them  for  a  moment j. 
and  enables  them  to  follow  their  bias.  Bu 
this  is  an  experiment  which  can  only  be  seen 
by  myself.  To  enable  you  to  see  it,  I  take  a 
pair  of  small  magnets  and  by  a  simple  optical* 
arrangement  throw  the  images  of  the  mag- 
net i  upon  the  screen.  Scattering  iron  filings 
over  the  glass  plate  to  which  the  small  magnets 
are  attached,  and  tapping  the  |>late,  you  see 
the  arrangement  of  the  iron  filings  in  those 
magnetic  curves  which  have  been  so  long 
familiar  to  scientific  men.* 


*Very  beautiful  specimens  of  these  curves  have 
been  recently  obtained,  and  fixed,  by  Prof.  Mayer, 
of  Hoboken. 


20 


SIX  LECTURES  ON  LIGHT. 


The  aspect   of  these  curves  so   fascinated 
Faraday  that  the  greater  portion  of  his  intel- 
lectual life   was   devoted   to  pondering  ove 
them.     He  invested  the  space  through  which 
they  run  with  a  kind  of  -materiality  ;  and  th 
probability  is,  that  the  progress  of  science  b) 
connecting   the    phenomena    of    magnetism 
with  the   luminiferous  ether,  will  prove  these 
"  lines  of  force,"  as  Faraday    loved   to    cal 
the  magnetic  curves,  to  represent  a  condition 
of  this   mysterious  substratum  of   all  radianl 
action. 

But  it  is  not  with  the  magnetic  curves,  as 
such,  that  I  now  wish  to  occupy  your  atten- 
tion ;  it  is  their  relationship  to  theoretic  con- 
ceptions that  we  have  now  to  consider.  By 
the  action  of  the  bar  magnet  upon  the  needle 
we  obtain  the  notion  of  a  polar  forcf  ;  by  the 
breaking  of  the  strip  of  magnetized  steel,  we 
attain  die  notion  that  polarity  can  attach 
itself  to  the  ultimate  particles  of  matter.  The 
experiment  with  the  iron  filings  introduces  a 
new  idea  into  the  mind  ;  the  idea,  namely,  of 
structural  arrangement.  Every  pair  of  filings 
possesses  four  poles,  two  of  which  arc  attrac- 
tive and  two  repulsive.  The  attractive  poles 
approach,  the  repulsive  poles  retreat  ;  the 
consequence  being  a  certain  definite  arrange- 
ment of  the  particles  with  reference  to  each 
other. 

Now,  this  idea  of  structure,  as  produced 
by  polar  force,  opens  a  way  for  the  intellect 
into  an  entirely  new  region,  and  the  reason  you 
are  asked  to  accompany  me  into  this  region 
is,  that  our  next  inquiry  relates  to  the  action 
of  crystals  upon  light.  Before  I  speak  of 
this  action,  I  wish  y^u  to  realize  the  process 
of  crystalline  architecture.  Look  then  into  a 
granite  quarry,  and  spend  a  few  minutes  in 
examining  the  rock.  It  is  not  of  perfectly 
uniform  texture.  It  is  rather  an  agglomera- 
tion of  pieces,  which,  on  examination,  pre- 
sent curiously-defined  forms.  You  have  there 
what  mineralogists  call  quartz,  you  have 
felspar,  you  have  mica.  In  a  mineralogical 
cabinet,  where  these  substances  are  preserve.! 
separately,  you  will  obtain  some  notion  of 
their  forms.  You  will  see  there,  also,  speci- 
mens of  beryl,  topaz,  emerald,  tourmaline, 
heavy  spar,  fluor-spar,  Iceland  spar — possibly 
a  full-formed  diamond,  as  it  quitted  the  hand 
of  .Nature,  not  yet  having  got  into  the  hands 
of  the  lapidary.  These  crystals,  you  will  ob- 
serve, are  put  together  according  to  law  ; 
they  are  not  chance  productions ;  and,  if 
you  care  to  examine  them  moro  minutely, 
you  will  find  their  architecture  capable  of 
being  to  some  extent  revealed.  They  split 
in  certain  directions  before  a  knife-edge,  ex- 
P"  sing  smooth  and  shining  surfaces,  which 
are  called  planes  of  cleavage  ;  and  by  follow- 
ing these  planes  you  sometimes  reach  an  in 
ternal  form,  disguised  beneath  the  external 
form  of  the  crystal.  Ponder  these  beautiful 
edifices  of  a  hidden  builder.  You  cannot 
help  asking  yourself  how  they  were  built  ; 
and  familiar  as  you  now  are  with  the  notion 


of  a  polar  force,  and  the  ability  of  that  force 
to  produce  structural  arrangement,  your  in- 
evitable answer  will  be,  that  those  crystals 
are  built  by  the  play  of  polar  forces  with 
which  their  ultimate  molecules  are  endowed. 
In  virtue  of  these  forces,  atom  lays  itself  to 
atom  in  a  perfectly  definite  way,  the  final 
visible  form  of  the  crystal  depending  upon 
this  play  of  its  molecules. 

Everywhere  in  Nature  we  observe  this 
tendency  to  run  into  definite  forms,  and 
nothing  is  easier  than  to  give  scope  to  this 
tendency  toy  artificial  arrangements.  Dis- 
solve nitre  in  water,  and  allow  the  water 
slowly  to  evaporate;  the  nitre  remains,  and 
the  solution  soon  becomes  so  concentrated 
that  the  liquid  form  can  no  longer  be  pre- 
served. The  nitre-molecules  approach  each 
other,  and  come  at  length  within  the  range 
of  their  polar  forces.  They  arrange  them- 
selves in  obedience  to  these  forces,  a  minute 
crystal  of  nitre  being  at  first  produced.  On 
this  crystal  the  molecules  continue  to  deposit 
themselves  from  the  surrounding  liquid.  The 
crystal  grows,  and  finally  we  have  large 
prisms  of  nitre,  each  of  a  perfectly  definite 
hape.  Alum  crystallizes  with  the  utmost 
ease  in  this  fash. on.  The  resultant  crystal 
s,  however,  different  in  shape  from  that  Ot 
nitre,  because  the  poles  of  the  molecules  are 
differently  disposed;  and,  if  they  be  only 
nursed  with  proper  care,  crystals  of  these 
substances  may  be  caused  to  grow  to  a  great 
ize. 

The  condition  of  perfect  crystallization  is, 
that  thi  crystallizing  force  shall  act  with  de- 
ibera^ion.  There  should  be  no  hurry  in  its 
operation;  but  every  molecule  ought  to  be 
Dermitted,  without  disturbance  from  its  neigh- 
>ors,  to  exercise  its  own  molecular  lights.  If 
he  crystallization  be  too  sudden,  the  regu- 
arity  disappears.  Water  may  be  saturated 
vith  sulphate  of  soda,  dissolved  when  the 
vater  is  hot,  and  afterward  permitted  to  cool. 
When  cold,  the  solution,  is  supersaturated  ; 
hat  is  to  say,  more  solid  matter  is  contained 
n  it  than  corresponds  to  its  temperature. 
Still  the  molecules  show  no  signs  of  building 
hemselves  together.  This  is  a  very  remaik- 
able,  though  a  very  common  fact.  The 
molecules  in  the  centre  of  the  liquid  are  so 
lampered  by  the  action  of  their  neighbors 
hat  freedom  to  fohow  their  own  tendencies 
s  denied  to  them.  Fix  your  mind's  eye  upon 
a  molecule  within  the  mass.  It  wishes  to 
unite  with  its  neighbor  to  the  right  but  it 
vishes  equally  to  unite  with  its  neighbor  to 
he  left  ;  the  one  tendency  neutralizes  the 
>ther,  ana  it  unites  with  neither.  We  have 
nere,  in  fact,  translated  into  molecular  action 
he  well-known  suspension  of  animal  volition 
jroduced  by  two  equally  inviting  bundles  of 
lay.  But,  if  a  crystal  of  sulphate  of  soda  be 
Iropped  into  the  solution,  the  molecular  in- 
lecision  ceases  On  the  crystal  the  adjacent 
molecules  will  immediately  precipitate  them- 
selves; on  these  again  others  will  be  precipi- 


SIX  LECTURES  ON  LIGHT. 


21 


)  and  this  act  of  precipitation  will  con- 
tinue from  the  top  of  the  flask  to  the  bottom, 
until  the  solution  has,  as  far  as  possible,  as- 
sumed the  solid  form.  The  crystals  here 
formed  are  small,  and  confusedly  arranged. 
The  process  has  been  too  hasty  to  a  imit  of 
the  pure  and  orderly  action  of  the  crystalliz- 
ing lorce.  It  typities  the  state  of  a  nation  in 
which  natural  and  healthy  change  is  resisted, 
until  society  becomes,  as  it  were,  supersatu- 
rated with  the  desire  lor  change,  the  change 
being  effected  through  confusion  and  revolu- 
tion, which  a  wise  foresight  might  have 
avoided. 

Let  me  illustrate  the  action  of  crystallizing 
force  by  two  examples  of  it  :  Nitre  might  be 
employed,  but  another  well-known  substance 
enables  me  to  make  the  experiment  in  a  bet- 
ter form.  The  substance  is  common  sal- 
ammoniac,  or  chloride  of  ammonium,  dis- 
solved in  water.  Cleansing  perfectly  a  glass 
plate,  the  solution  of  the  chloride  is  poured 
over  the  glass,  to  which,  when  the  plate  is  set 
on  edge,  a  thin  film  of  the  liquid  adheres. 
Warming  the  glass  slightly,  evaporation  is 
promoted  ;  the  plate  is  then  placed  in  a  solar 
microscope,  and  an  image  of  the  film  is  thrown 
upon  a  white  screen.  The  warmth  of  the  il- 
luminating beam  adds  itself  to  that  already 
impa-  ted  to  the  glass  plate,  so  that  after  a 
moment  or  two  the  film  can  no  longer  exist  in 
the  liquid  condition.  Molecule  then  closes 
with  molecule,  and  you  have  a  most  impres- 
sive display  of  crystallizing  energy  overspread- 
ing the  whole  screen.  You  may  produce 
something  similar  if  you  breathe  upon  the 
frost  ferns  which  overspread  your  window- 
panes  in  winter,  and  then  observe  through  a 
lens  the  subsequent  recongelation  of  the  film. 

Here  the  crystallizing  force  is  hampered  by 
the  adhesion  of  the  film  to  the  glasys ;  never- 
theless, the  play  of  power  is  strikingly  beau- 
tiful. Sometimes  the  crystals  start  from  the 
edge  of  the  film  and  run  through  it  from  that 
edge,  for,  the  crystallization  being  once 
started,  the  molecules  throw  themselves  by 
preference  on  the  crystals  already  formed. 
Sometimes  the  crystals  start  from  definite 
nuclei  in  the  centre  of  the  film  ;  every  small 
crystailins  particle  which  rests  in  the  film  fur- 
nishes a  starting-point.  Throughout  the  pro- 
cess you  notice  one  feature  which  is  perfectly 
unalterable,  and  that  is,  angular  magnitude. 
The  spiculce  branch  from  the  trunk,  and  from 
these  branches  others  shoot ;  but  the  angles 
enclosed  by  the  spiculre  are  unalterable.  In 
like  manner  you  may  find  alum-crystals, 
quartz-crystals,  and  all  other  crystals,  dis- 
torted in  shape.  They  are  thus  far  at  the 
mercy  of  the  accidents  of  crystallization  ;  but 
in  one  particular  they  assert  their  superiority 
over  ail  such  accidents — angttlar  magnitude 
is  always  rigidly  preserved. 

My  second  example  of  the  action  of  crys- 
tallizing force  is  this:  !  y  sending  a  voltaic 
current  through  a  liquid,  you  know  that  we 
decompose  the  liquid,  and  if  it  contains  a 


metal,  we  liberate  this  metal  by  the  electro- 
lysis. This  small  cell  contains  a  solution  of 
acetate  of  lead,  and  this  substance  is  chosen 
because  lead  lends  itself  freely  to  this  crys- 
tallizing power.  Into  the  cell  dip  two  very 
thin  platLum  wires,  and  these  are  connected 
by  other  wires  with  a  small  voltaic  battery. 
On  sending  the  voltaic  current  through  the 
solution,  the  1  ad  will  be  sl.wly  severed  from 
the  atoms  with  which  it  is  now  combined;  it 
will  be  liberated  upon  one  of  the  wires,  and 
at  the  moment  of  its  liberation  it  will  obey 
the  polar  forces  of  its  atoms,  and  produce 
crystalline  forms  of  exquisite  beauty.  They 
are  now  before  you,  sprouting  like  ferns 
from  the  wire,  appearing  indeed  like  vegeta- 
ble growths  rendered  so  rapid  as  to  be  plain- 
ly visible  to  the  naked  eye.  On  reversing  the 
current,  these  wonderful  lead-fror  ds  will  dis- 
solve, while  from  the  other  wire  filaments  of 
lead  dart  through  the  liquid.  In  a  moment1 
or  two  the  growth  of  the  lead-trees  recom- 
mences, but  they  now  cover  the  other  wire. 
In  the  process  of  crystallization,  Nature  first 
reveals  herself  as  a  builder.  Where  do  her 
operations  stop  ?  Does  she  continue,  by  the 
play  of  the  same  forces,  to  form  the  vegeta- 
ble, and  afterwards  the  animal  ?  Whatever 
the  answer  to  these  questions  may  be,  trust 
me  that  the  notions  of  the  coming  genera- 
lions  regarding  this  mysterious  thing,  which 
some  have  called  "brute  matter,"  will  be 
very  different  from  those  of  the  generations 
past. 

There  is  hardly  a  more  beautiful  and  in- 
structive example  of  this  play  of  molecular 
force  ti  an  that  furnished  by  the  case  of  water. 
You  have  seen  the  exquisite  fern-like  forms 
produced  by  the  crystallization  of  a  film  of 
water  on   a  cold  window  pane.      You    have 
also  probably  noticed  the  beautiful  rosettes 
tied  together  by  the  crystallizing  force  during 
the  descent  of  a  snow-shower  on  a  very  calm 
day.     The  slopes  and  summits  of   the  Alps 
are  loaded  in  winter  with  these  blossoms  of 
the  frost.      They  vary  infinitely  in  detail  of 
beauty,  but  the  same  angular  magnitude  is 
preserved  throughout.     An  inflexible  power 
binds  spears  and  spiculce  to  the  angle  of  60 
degrees.     The  common  ice  of  our  lakes  ii 
also  ruled  ki  its  deposition  by  the  same  angle. 
You   may  sometimes   see   in  freezing  water 
small  crystals  of  stellar  shapes,  each  star  con- 
sisting of  six  rays,  with  this  angle  of  60°  be- 
tween every  two  of  them.      'i  his  structure 
may  be  revealed  in  ordinary  ice.      In  a  su<fc- 
j  beam,  or,  failing  that,  in  our  electric  beam, 
I  we  have  an  instrument  delicate  enough  to 
I  unlock  the  frozen  molecules  without  disturb- 
|  ing  the  oider  of  their  architecture.      Cutting 
I  from  clear,  sound,  regularly-frozen  ice  a  sl-tb 
i  parallel  to  the  planes  of  freezing,  and  send- 
I  ing  a  sunbeam  through  such  a  slab,  it   lique- 
j  fies  internally  at  special  points,  round  each 
!  point  a  six-petalled  liquid  flower  of  exquisite 
I  beauty  being  formed.     Crowds  of  such  flow?' 
'  ers  are  thus  produced. 


22 


SIX  LECTURES  ON  LIGHT. 


A  moment's  further  devotion  to  the  crys- 
tallization of  water  will  be  well  repaid  ;  for 
the  sum  of  qualities  which  renders  ;his  sub- 
stance fitted  to  play  its  part  in  Nature  may 
well  excite  wonder  and  stimulate  thought. 
Like  almost  all  other  substances,  water  is  ex- 
panded by  heat  and  contracted  by  cold.  Let 
this  expansion  and  contraction  be  first  illus- 
trated : 

A  small  fla  1:  is  filled  with  colored  water, 
and  stopped  with  a  cork.  Through  the  cork 
passes  a  glass  tube  water-tight,  the  liquid 
standing  at  a  Certain  height  (/',  Fig.  7)  in  the 
tube.  The  flask  and  its  tube  resemble  the 
bulb  and  stem  of  a  thermometer.  Applying 
the  heat  of  a  spirit-lamp,  the  water  rises  in 
the  tube,  and  finally  trickles  over  the  top  (/). 
Expansion  by  heat  is  thus  illustrated. 


the  definite  temperature  of  39°  Fahr.  Crys- 
tallization has  virtually  here  commenced,  the 
molecules  preparing  themselves  for  the  subse- 
quent act  of  solidification  which  occurs  at 
32°,  and  in  which  the  expansion  suddenly 
culminates.  In  virtue  of  this  expansion, 
ice,  as  you  know,  is  lighter  than  water  in  the 
proportion  of  8  to  9.* 

It  is  my  desire,  in  these  lectures,  to  lead 
you  as  closely  a*  possible  to  trie  limits 
hitherto  attained  by  scientific  thought,  and, 
in  pursuance  rf  this  desire,  I  have  now  to 
invite  your  attention  to  a  molecular  problem 
of  great  interest,  but  of  great  complexity.  I 
wish  you  to  obtain  sue  i  an  insight  of  the 
molecular  world  as  shall  give  the  intellect 
satisfaction  when  reflecting  on  the  deport- 
n:ent  of  water  before  and  during  the  act  of 


FIG.  7. 

Projection  of  experiment  :    E  is  the  nozzle  of  the  lamp,  L  a  converging  lens,  and  /  i  the  image  of  the  liquid 
column. 


Removing  the  lamp  and  piling  a  freezing 
mixture  in  the  vessel  (B)  round  the  flask,  the 
liquid  column  falls,  thus  -  showing  the  con- 
traction of  the  water  by  the  cold.  But  let 
the-  freezing  mixture  continue  to  act :  the 
tailing  of  the  column  continues  to  a  certain 
point  ;  it  then  ceases.  The  top  of  tne  col- 
umn remains  stationary  for  son-e  seconds, 
and  afterwards  begins  to  rise.  The  contrac- 
tion has  ceased,  and  expansion  by  cold  sets  in. 
Let  the  expansion  continue  till  the  liquid 
trickles  a  second  time  over  the  top  of  the 
tube.  The  freezing  mixture  has  here  pro- 
duced to  all  appearance  the  same  effect  as  the 
flame.  In  the  case  of  water,  contraction  by 
cold  ceases  and  expansion  by  cold  sets  in  at 


crystallization.      Consider,    then,    the     ideal 
case  of  a  number  of   magnets    deprived   of 


*  In  a  little  volume  entitled  "  Forms  of  Water," 
I  have  mentioned  that  cold  iron  floats  upon  molten 
iron.  In  company  with  my  friend  Sir  William  Arm- 
strong, I  had  repeated  opportunities  of  witnessing 
this  fact  in  his  works  at  Elswick,  in  1863.  Faraday, 
I  remember,  spoke  to  me  subsequently  of  the  com- 
pleteness of  iron  castings  as  probably  due  to  the 
swelling  of  the  metal  on  solidification.  Beyond  this, 
I  have  giv^n  the  subject  no  special  attention,  and  I 
know  that  many  intelligent  iron-founders  doubt  the 
fact  of  expansion.  It  is  quite  possible  that  the  solid 
floats  because  it  is  not  wetted  by  the  molten  iron,  its 
volume  being  virtually  augmented  by  capillary  re- 
pulsion. Certain  flies  walk  freely  upon  water  in  vir- 
tue of  an  action  of  this  kind.  With  bismuth,  how- 
ever, it  is  easy  to  burst  iron  bottles  by  the  force  of 
solidification. 


SIX  LECTURES  ON  LIGHT. 


23 


weight,  but  retaining  thtir  polar  forces.  If 
we  had  a  liquid  of  the  specific  gravity  of 
steel,  we  might,  by  making  the  magnets 
float  in  it,  realize  this  state  of  things,  for  in 
such  a  liquid  the  magnets  would  neither  sink 
nor  swim.  Now,  the  principle  of  gravitation 
is  .that  every  particle  of  matter  attracts  every 
other  particle  with  a  force  varying  as  the  in- 
inverse  square  of  the  distance.  In  virtue  of 
the  attraction  of  gravity,  then,  the  magnets, 
if  perfectly  free  to  move,  would  slowly  ap- 
proach each  other. 

But  besides  the  unjsolar  force  of  gravity, 
which  belongs  to  matter  in  general,  the  mag- 
nets are  endowed  with  the  polar  force  of 
magnetism.  For  a  time,  however,  the  polar 
forces  do  not  sensibly  come  into  play.  In 
this  condition  the  magnets  resemble  our  water 
molecules  at  the  temperature  say  of  50". 

E 


;  with  the  force  of  contraction  until  the  freezing 
j  temperature  is  attained.  Here  the  polar 
|  forces  suddenly  and  finally  gain  the  victory. 
{  The  molecules  close  up  and  form  solid  crys- 
I  tals,  a  considerable  augmentation  of  volume 
|  being  the  immediate  consequence. 

\Ve  can  still  further  satisfy  the  intellect  by 
I  showing  that  these  conceptions  can  be  real- 
ized by  a  model.  The  molecule  of  water  is 
composed  of  two  atoms  of  hydrogen,  united 
to  one  of  oxygen.  We  may  assume  the  mole- 
cule built  up  of  these  atoms  to  be  pyramidal. 
Suppose  the  triangles  in  Fig.  8  to  be  drawn 
touching  the  sides  of  the  molecule,  and  the 
disposition  of  the  polar  forces  to  be  that  indi- 
cated by  the  letters  ;  the  points  marked  A 
being  attractive,  and  those  marked  R  repel- 
lent. In  virtue  of  the  general  attraction  of 
the  molecules,  let  them  be  drawn  towards  the 


But  the  magnets  come  at  length  sufiici"Tily 
near  each  other  to  enable  their  poles  to  tiler- 
act.  From  this  point  the  action  cease*  to  be 
a  general  attraction  of  the  masses.  An  at- 
traction of  special  points  of  the  masses  and  a 
repulsion  of  other  points  now  come  into  play; 
and  it  is  easy  to  see  that  the  rearrangement 
of  the  magnets  consequent  upon  the  intro- 
duction of  these  new  forces  may  be  suth  as 
to  require  a  greater  amount  of  room.  This, 
I  take  it,  is  the  case  with  our  water-mole- 
cules. Like  the  magnets,  they  approach  each 
other  as  wholes ^  until  the  temperature  ^9°  is 
reached.  Previous  to  this  temperature, 
doubtless,  the  polar  forces  had  begun  to  act, 
and  at  this  temperature  their  action  exactly 
balances  the  contraction  due  to  cold.  At 
lower  temperatures  the  polar  forces  predomi- 
nate. But  they  carry  on  a  gradual  struggle 


positions  marked  by  the  full  lines,  and  then 
suppose  the  polar  attractions  and  repulsions 
to  act.  A  will  turn  towards  A',  and  R  will 
retreat  from  Rx.  The  molecules  will  be  caused 
lo  lotate,  their  final  positions  being  that  shown 
by  the  dotted  lines.  But  the  circle  surround 
ing  the  latter  is  larger  than  that  surrounding 
the  full  lines,  which  shows  that  the  molecules 
in  their  new  positions  require  more  room.  In 
this  v.ay  we  obtain  an  image  of  the  molecular 
mechai  ism  active  in  the  case  of  water.  The 
demand  for  more  room  is  made  with  an  energy 
sufficient  to  overcome  all  ordinary  resistances. 
Your  lead  pipes  yield  readily  to  this  power;  but 
iron  does  the  same,  and  bomb-shells,  as  you 
know,  can  be  burst  by  the  freezing  of  water. 
Thick  iron  bottles  filled  with  water  and  placed 
in  a  freezing  mixture  are  shivered  into  frag- 
ments by  the  resistless  vigor  of  molecular  force. 


24 


SIX  LECTURES  ON  LIGHT. 


We  have  now  to  exhibit  the  bearings  of 
crystallization  upon  optical  phenomena.  Ac- 
cording to  the  undulatory  theory,  the  velocity 
of  light  in  water  and  glass  is  less  than  in 
air.  Consider,  then,  a  small  portion  of  a 
wave  issuing  from  a  point  of  light  so  distant 
that  the  portion  may  be  regarded  as  practi- 
cally straight.  Moving  vertically  downwards, 
and  impinging  on  an  horizontal  surface  of 
glass,  the  wave  would  go  through  the  glass 
without  change  of  direction.  But,  as  the 
velocity  in  glass  is  less  than  the  velocity  in 
air,  the  wave  would  be  retarded  on  passing 
into  the  denser  medium. 

But  suppose  the  wave,  before  reaching  the 
glass,  to  be  oblique  to  the  surface  ;  that  end 
of  the  wave  which  first  reaches  the  glass  will 
be  the  first  retarded,  the  other  portions  as 
they  enter  the  glass  being  retarded  in  succes- 
sion. This  retardation  of  the  one  end  of  the 
wave  causes  it  to  swing  round  and  change  its 
front,  so  that  when  the  wave  has  fully  entered 
the  glass  its  course  is  oblique  to  its  original  di- 
rection. According  to  the  undulatory  theory, 
light  is  thus  refracted. 


In  water,  fcr  example,  there  is  nothing  in 
the  grouping  of  the  molecules  to  interfere 
with  the  perfect  homogeneity  of  the  ether ; 
but,  when  water  crystallizes  to  ice,  the  case 
is  different.  In  a  plate  of  ice  the  elasticity 
of  the  ether  in  a  direction  perpendicular  to 
the  surface  of  freezing  is  different  from  what 
it  is  parallel  to  the  surface  of  freezing  ;  ice  is. 
therefore,  a  double  refracting  substance. 
Double  refraction  is  displayed  in  a  particu- 
larly impressive  manner  by  Iceland  spar, 
which  is  crystallized  carbonate  of  lime.  The 
difference  of  ethereal  density  in  two  direc- 
tions in  this  crystal  is  very  great,  the  separa- 
tion oi  the  beam  into  the  two  halves  being, 
therefore,  particularly  striking. 

Before  you  is  now  projected  an  image  of  our 
carbon-points.  Introducing  the  spar,  the  beam 
which  builds  the  image  is  permitted  to  pass 
through  it;  instantly  you  have  the  single  image 
divided  into  two.  Projecting  an  image  ot  the 
aperature  through  which  the  light  issues  from 
the  electric  lamp,  and  introducing  the  spar, 
two  luminous  ti  k;,  insteadof  one,  appear 
immediately  upon  the  screen.  (See  Fig.  Q.X 


FIG.  9. 


The  two  elements  of  rapidity  of  propaga- 
tion, both  of  sound  and  light,  in  any  sub- 
stance whatever,  are  elasticity  and  density, 
and  the  enormous  velocity  of  light  is  attain- 
able because  the  ether  is  at  the  same  time  of 
infinitesimal  density  and  of  enormous  elas- 
ticity. It  surrounds  the  atoms  of  all  bodies, 
but  seems  to  be  so  acted  upon  by  them  that 
its  density  is  increased  without  a  proportionate 
increase  of  elasticity  ;  this  would  account  for 
the  diminished  velocity  of  light  in  refracting 
bodies.  In  virtue  of  the  crystalline  archi- 
' lecture  that  we  have  been  considering,  the 
ether  in  many  crystals  possesses  different  j 
densities  in  different  directions  ;  and  the  con- 
sequence is,  that  some  of  these  media  trans- 
mit light  with  two  different  velocities.  Now, 
refraction  depends  wholly  upon  the  change 
of  velocity  on  entering  the  refracting  medium  ; 
and  is  greatest  where  the  change  of  volicity 
is  greatest.  Hence,  as,  in  many  crystals,  we 
have  two  different  velocities,  we  have  also 
two  different  refractions,  a  beam  of  light  being 
divided  by  such  crystals  into  two.  This  ef- 
fect U€attad<tifai&£  refraction. 


The  two  beams  into  which  the  spar  divides 
the  single  incident-beam  do  not  behave  alike. 
One  of  them  obeys  the  ordinary  law  of  re- 
fraction discovered  by  Snell,  and  this  is 
called  the  ordinary  ray.  The  other  does  not 
obey  the  ordinary  law.  Its  index  of  refrac- 
tion, for  example,  is  not  constant,  nor  do  the 
incident  and  refracted  rays  always  lie  in  the 
same  plane.  It  is,  therefore,  called  the  ex- 
traordinary ray.  Pour  water,  and  bisulphide 
of  carbon  into  two  cups  of  the  same  depth  ; 
looked  at  through  the  liquid,  the  cup  that  con- 
tains the  more  strongly-refracting  liquid  will 
appear  shallower  than  the  other.  Place  a 
piece  cf  Iceland  spar  over  a  dot  of  ink  ;  two 
dots  are  seen,  but  one  appears  nearer  than  the 
other.  The  nearest  dot  belongs  to  the  most, 
strongly-refracted  ray,  which  in  this  case  is 
ths  ordinary  ray.  Turn  the  spar  round,  and 
the  extraordinary  image  of  the  spot  rotates 
roi:nd  the  ordinary  one. 

The  double  refraction  of  Iceland  spar  was 
first  treated  in  a  work  published  by  Erasmus 
Bartholimus,  in  1669.  The  celebrated  Huy- 
ghens  sought  to  account  for  the  phenomenon 


SIX  LECTURES  ON  LIGHT. 


on  the  principles  of  the  wave  theory,  and  he 
succeeded  in  doirg  so.  He  made  highly  im- 
portant observations  on  the  distinctive  charac- 
ter of  the  two  beams  transmitted  by  the  spar. 
Newton,  reflecting  on  the  observations  of 
Huyghens,  came  to  the  conclusion  that  each 
of  the  beams  had  two  sides  ;  and  from  the 
analogy  of  this  two  sidedness  with  the  two 
tndedness  of  a  magnet,  wherein  consists  its 
polarity,  the  two  beams  came  subsequently  to 
be  described  as  polarized. 

We  shall  study  this  subject  of  \b&  polariza- 
tion of  light  with  great  ease  and  profit  by 
means  of  a  crystal  of  tourmaline.  But  let  us 
start  with  a  clear  conception  of  an  ordinary 
beam  of  light.  It  has  been  already  explained 
that  the  vibrations  of  the  individual  ether- 
particles  are  executed  across  the  line  of  prop- 
agation. In  the  case  of  ordinary  light  we 
are  to  figure  the  ether  particles  as  vibrating 
in  all  directions,  or  azimuths,  as  it  is  some- 
times expressed,  across  this  line. 

Now,  in  a  plate  of  tourmaline  cut  parallel 
to  the  axis  of  the  crystal,  the  beam  of  incident 
light  is  divided  into  two,  the  one  vibrating 
parallel  to  the  axis  of  the  crystal,  the  other  at 
right  angles  to  the  axis.  The  grouping  of 
the  molecules,  and  of  the  ether  associated 
with  the  molecules,  reduces  all  the  vibrations 
incident  upon  the  crystal  to  these  two  direc- 
tions. One  of  these  beams,  namely  that  one 
whose  vibrations  are  perpendicular  to  the 
axis,  is  quenched  with  exceeding  rapidity  by 
the  tourma  ine,  so  '.hat,  after  having  passed 
through  a  very  small  thickness  of  the  crystal, 
the  light  emerges  with  all  its  vibrations  re- 
duced to  a  single  plane.  In  this  condition  it 
is  what  we  call  a  beam  of  plane  polarized 
light. 

A  moment's  reflection  will  show,  if  what 
has  been  stated  be  correct,  that,  on  placing 
a  second  plate  of  tourmaline  with  its  axis 
parallel  to  the  first,  the  light  will  pass  through 
both  ;  but  that,  if  the  axes  be  crossed,  the 


FIG.   10. 


light  that  passes  through  the  one  plate  will 
be  quenched  by  the  other,  a  total  interception 


of  the  light  being  the  consequence.  The 
image  of  a  plate  of  tourmaline,  1 1  (Fig.  10), 
is  now  before  you.  I  place  parallel  to  it 
another  plate,  t'  tf :  the  green  of  the  crystal 
is  a  little  deepened,  nothing  more.  By  means 
of  an  endless  screw,  I  now  turn  one  of  the 
crystals  gradually  round  ;  as  long  as  the  two 
plates  are  'oblique  to  each  other,  a  certain 
portion  of  light  gets  through  ;  but,  when  they 
are  at  right  angles  to  each  other,  the  space 
common  to  both  is  a  space  of  darkness,  as 
shown  in  Fig.  n. 

Let  us  return  to  a  single  plate  ;  and  let  me 
say  that  it  is  on  the  green  light  transmitted 
by  the  tourmaline  that  you  are  to  fix  your  at- 
tention. We  have  now  to  illustrate  the  two- 
sidedness  of  that  green  light.  The  light  sur- 
rounding the  green  image  being  ordinary 
light,  is  reflected  by  a  plane  glass  mirror  in  i 
all  directions ;  the  green  light,  on  the  con- 
trary, is  not  so  reflected.  The  image  of  the 
tourmaline  is  now  horizontal  ;  reflected  up- 
wards, it  is  still  green  ;  reflected  sideways, 
the  image  is  reduced  to  blackness,  because  of 
the  incompetency  of  the  green  1'ght  to  be  re- 
flected in  this  direction.  Making  the  plate 
of  tourmaline  vertical  and  reflecting  it  as 
before,  in  the  upper  image  the  light  is 
quenched  ;  in  the  side  image  you  have  now 
the  green.  Picture  the  thing  clearly.  In 
the  one  case  the  mirror  receives  the  impact 
ot  the  edges  of  the  waves,  ana  the  green  light 
is  quenched.  In  the  other  case  the  sides  of 
the  waves  strike  the  mirror,  and  t  e  green 
light  is  reflected.  To  render  the  extinction 
complete,  the  light  must  be  received  upon 
the  mirror  at  a  special  angle.  What  this 
angle  is  we  shall  learn  presently. 

The  quality  of  two-sidedness  conferred 
upon  light  by  crystals  may  also  be  conferred 
upon  it  by  ordinary  reflection.  Malus  made 
this  discovery  in  1808,  while  looking  through 
Iceland  spar  at  the  light  of  the  sun  reflected 
from  the  windows  of  the  Luxembourg  palace 
in  Paris.  I  receive  upon  a  plate  of  window- 
glass  the  beam  from  our  lamp  ;  a  great  por- 
tion of  the  light  reflected  from  the  glass  is 
polarized  ;  the  vibrations  of  this  reflected 
beam  are  executed,  for  the  most  part,  paral- 
lel to  the  surface  of  the  glass,  and,  if  the 
glass  be  held  so  that  the  beam  shall  make  an 
angle  of  58°  with  the  perpendicular  to  the 
glass,  the  whole  of  the  reflected  beam  is  polar- 
ized. It  was  at  this  angle  that  the  image 
of  the  tourmaline  was  completely  quenched 
in  our  former  experiments.  It  is  called  the 
polarizing  angle. 

And  now  let  us  try  to  make  substantially 
the  experiment  of  Malus.  I  receive  the  beam 
from  the  lamp  upon  this  plate  of  glass  and 
reflect  it  through  the  spar.  Instead  of  two 
images,  you  see  but  one.  So  that  the  light, 
when  polarized,  as  it  now  is,  can  only  get 
through  the  spar  in  one  direction,  and  conse- 
quently produce  <but  one  image.  Why  is 
this  ?  In  the  Iceland  spar,  as  in  the  tour- 
maline, all  the  vibrations  of  the  ordinary  light 


26 


SIX  LECTURES  ON  LIGHT. 


are  reduced  to  two  planes  at  right  angles  to 
each  other  ;  but,  unlike  the  tourmaline,  both 
beams  are  transmitted  with  equal  facility  by 
the  spar.  The  two  beams,  in  short,  emerg- 
ent from  the  spar  are  polarized,  their  direc- 
tions of  vibration  being  at  right  angles  to 
each  other.  When,  therefore,  the  light  was 
Dolarized  by  reflection,  the  direction  of  vibra- 
tion in  the  spar  which  corresponded  to  the 


conclude?  That  the  green  light  will  be 
transmitted  along  the  latter,  which  is  parallel 
to  the  tourmaline,  a:id  not  along  the  former, 
which  is  perpendicular  to  it.  Hence  we  may 
infer  that  one  image  of  the  tourmaline  will 
show  the  ordinary  green  light  cf  the  crystal, 
while  the  other  image  will  be  black.  Let  us 
test  our  reasoning  by  experiment  :  it  is  veri- 
fied to  the  letter.  (Fig.  13.) 


FIG 


direction  of  vibration  of  the  pokrized  beam 
transmitted  it,  and  that  direction  only.  But 
one  image,  therefore,  was  possible  under  the 
conditions. 

And  now  you  have  it  in  your  power  to 
check  many  of  my  statements,  and  you  will 
observe  that  such  logic  as  connect  our  experi- 
ments is  simply  a  transcript  of  the  logic  of 
Nature.  On  the  screen  before  you  arc  the 


Let  us  push  our  test  still  further.  By 
means  of  an  endless  screw,  the  crystal  can 
l*e  turned  ninety  degrees  round.  The  black 
image,  as  I  turn,  becomes  gradually  brighter 
and  the  bright  one  gradually  darker;  at  an 
angle  cf  forty- five  degrees  both  images  are 
equally  bright  (Fig.  13);  while,  when  ninety 
degrees  have  been  obtained,  the  axis  cf  the 
crystal  being  then  vertical,  the  briglu  and 


FIG 

two  disks  of  light  produced  by  the  double  re- 
fraction of  the  spar.  They  are,  as  you 
know,  two  images  of  the  aperture  through 
which  the  light  issues  from  the  camera. 
Placing  the  tourmaline  in  front  of  the  aper- 
ture, two  images  of  the  crystal  will  be  ob- 
tained ;  but  now  let  us  reason  out  what  is  to 
be  expected  from  this  experiment.  The  light 
emergent  from  the  tourmaline  is  polarized. 


black  ftnages    have    changed   places.     (Fig. 
I4-) 

Given  two  beams  transmitted  through  Ice- 
land spar,  it  is  perfectly  manifest  that  we 
have  it  in  our  power  to  determine  instantly, 
by  means  of  a  plate  of  tourmaline,  the  direc- 
tions in  which  the  ether-particles  vibrate  in 
the  two  beams.  I  might  place  the  double- 
refracting  spar  in  any  position  whatever.  A 


FIG.  14. 


Placing  the  crystal  with  its  axis  horizontal, 
the  vibrations  of  the  transmitteu  light  will  be 
horizontal.  Now  the  spar,  as  already  stated, 
has  two  perpendicular  directions  ot  vibration, 
one  of  which,  at  the  present  moment,  is  ver- 
tical, the  other  horizontal.  "What  are  we  to 


minute's  trial  with  the  tourmaline  would 
enable  you  to  determine  the  position  which 
yields  a  black  and  a  bright  image,  and  from 
these  you  would  at  once  infer  the  directions 
of  vibration. 

Further,    the  two  beams  from  the    spar 


SIX  LECTURES  ON  LIGHT. 


27 


being  thus  polarized,  if  they  be  suitably  re- 
ceived upon  a  plate  of  glass  at  the  polarizing 
angle,  one  of  them  will  be  reflected,  the 
other  not.  This  is  the  conclusion  of  reason 
from  our  previous  knowledge;  but  you  ob- 
serve that  reason  is  justified  by  experiment. 
(Figs.  15  and  16.) 

I  have  said  that  the  whole  of  the  beam  re- 
flected from  glass  at  the  polarizing  angle  is 
polarized;  a  word  must  now  be  added  regard- 
ing the  larger  portion  of  the  light  transmitted 
by  the  glass.  The  transmitted  beam  contains 
a  quantity  of  polarized  light  equal  to  that  of 
tie  reflected  beam;  but  this  quantity  is  only 
a  fi action  of  the  whole  transmitted  light.  By 
taking  two  plates  of  glass  instead  cf  one,  we 


(B  is  the  birefracting  spar,  dividing  the  incident 
li^fit  into  the  two  beams,  o  and  e.  G  is  the  mirror). 
Tne  beam  is  here  reflected  laterally.  When  the  re- 
flection is  ufitvards^  the  other  beam  is  reflected,  as 
shown  in  Fig.  16. 


augment  the  quantity  of  the  transmitted  polar- 
ized light;  and, by  taking  a  bundle  of  plates,  we 


so  increase  the  quantity  as  to  render  the  trans- 
mitted beam,  for  all  practical  purposes, per- 
fectly polarized.  Indeed,  bundles  of  glass 
plates  are  often  cmp'oyed  as  a  means  of  fur- 
nishing polarized  light. 

One  word  more.  When  the  tourmalines 
are  crossed,  the  space  where  they  cross  each 
other  is  black.  But  we  have  seen  that  the 
least  obliquity  on  the  part  of  the  crystals  per- 
mits light  to  get  through  both.  Now  sup- 
I  ose,  when  the  two  plates  are  crossed,  that 
we  interpose  a  third  plate  of  tourmaline  be- 
tween them,  wiih  its  axis  oblique  to  both.  A 
portion  of  the  light  transmitted  by  the  first 
plate  will  get  through  this  intermediate  cne. 
But,  after  it  has  got  through,  its  plane  of  vi- 
bration is  changed:  it  is  no  longer  perpendicu- 
lar to  the  axis  cf  the  crystal  in  front.  Hence 
it  will  get  thiough  that  crystal.  Thus,  by 
reasoning,  we  infer  that  the  interposition  of  a 
third  plate  of  tourmaline  will  in  part  abolish 
the  darkness  produced  by  the  perpendicular 
crossing  of  the  other  two  plates.  I  have  not 
a  third  plate  of  tourmaline  ;  but  the  talc  or 
mica  which  you  employ  in  your  stoves  is  a 
more  convenient  substance,  which  acts  in  the 
same  way.  Between  the  crossed  tourmalines 
I  introduce  a  film  of  this  crystal.  You  see 
the  edge  of  the  film  slowly  descending,  and 
as  it  descends  between  the  tourmalines,  light 
takes  the  place  cf  darkness.  The  darkness, 
in  factj  setmed  scraped  away  as  if  it  were 
something  material.  This  effect  has  been 
called — and  improperly  called — depolarization. 


LECTURE   IV. 

Chromatic  Phenomena  produced  by  Crystals  on  Polar- 
ized Light:  The  Nicol  Prism  :  Polarizer  and  Ana- 
lyzer: Action  of  thick  and  thin  Plates  of  Selenite: 
Colors  dependent,  on  Thickness:  Resolution  of  Po- 
larized Beam  into  two  others  by  the  Selenite  ;  One 
of  them  more  retarded  than  the  other:  Recom- 
pounding  of  the  two  Systems  of  Waves  by  the  Ana-r 
lyzer:  Interference  thus  rendered  possible  :  Conse- 
quent Production  of  Colors:  Action  of  Bodies 
Mechanically  strained  or  pressed  :  Action  of  Sono- 
rous Vibrations:  Action  of  Glass  strained  or  pressed 
by  Heat:  Circular  Polarization:  Chromatic  Phe- 
nomena produced  by  Quartz  :  The  Magnetization 
of  Light:  Rings  surrounding  the  Axes  of  Crystals: 
Blaxal  and  Uniaxal  Crystals :  Grasp  of  the  Undu- 
latory  Theory. 

We  now  stand  upon  the  threshold  of  a  new 
and  splendid  optical  domain.  We  have  to 
examine,  this  evening,  the  chromatic  phe- 
nomena produced  by  the  action  ot  crystals, 
and  double-refracting  bodies  gene: ally,  upon 
polarized  light.  For  a  long  time  investigators 
were  compelled  to  employ  plates  of  tourmaline 
for  this  purpose,  and  the  progress  they  made 
with  so  defective  a  means  of  inquiry  is  aston- 
ishing. But  these  men  had  their  hearts  in 
their  work,  and  were  on  this  account  enabled 
to  extract  great  results  from  small  instrumen- 
tal appliances.  But  we  have  better  apparatus 
now.  You  have  seen  the  two  beams  emer- 
gent from  Iceland  spar,  and  have  proved 
them  to  be  polarized.  If  we  could  abolish 


SIX  LECTURES  ON  LIGHT. 


one  of  these  beams,  we  might  employ  the 
other  for  experiments  on  polarized  light. 

These  beams,  as  you  know,  are  refracted 
differently,  and  from  this  we  are  able  to  infer 
that  under  some  circumstances  the  one  may 
be  totally  reflected,  and  the  other  not.  An 
optician,  named  Nicol,  cut  a  crystal  of  Ice- 
land spar  in  two  in  a  certain  direction.  He 
polished  the  severed  urfaces,  and  reunited 
them  by  Canada  balsam,  the  surface  of  union 
being-  so  inclined  to  the  beam  traversing  the 
spar  that  the  ordinary  ray,  which  is  the  most 
highly  refracted,  was  totally  reflected  by  the 
balsam,  while  the  extraordinary  ray  was  per- 
mitted to  pass  on.  The  invention  of  the 
Nicol  prism  was  a  great  step  in  practical  op- 
tics,  and  quite  recently  such  prisms  have 
been  constructed  of  a  size  which  enables 
audiences  like  the  present  to  witness  the 
chromatic  phenomena  of  polarized  light  to  a 
degree  altogether  unattainable  a  short  time 
ago.  The  two  prisms  here  before  you  belong 
to  my  excellent  friend,  Mr.  William  Spottis- 
woode,  and  they  were  manufactured  by  Mr. 
Ladd.  I  have  with  me  another  pair  of  very 
noble  prisms,  still  larger  than  these,  manu- 
factured for  me  by  Mr.  Browning,  who  has 
gained  so  high  and  well-merited  a  reputation 
in  the  construction  of  spectroscopes. 

These  two  Nicol  prisms  play  the  same 
part  as  the  crystals  of  tourmaline.  Placed 
with  their  directions  of  vibration  parallel, 
the  light  passes  through  both.  When  these 
directions  are  crossed,  the  light  is  quenched. 
Introducing  a  film  of  mica  between  the 
prisms,  the  light  is  in  part  restored.  But 
notice,  when  the  film  of  mica  is  thin,  you 
have  sometimes  not  only  light,  but  colored 
light.  Our  work  for  some  time  to  come  will 
be  the  examination  of  these  colors.  With 
this  view,  I  will  take  a  representative  crystal, 
one  easily  dealt  with;  the  crystal  gypsum,  or 
selenite,  which  is  crystallized  sulphate  of 
lime.  Between  the  crossed  Nicols  I  place  a 
thick  plate  of  this  crystal;  like  the  mica,  it 
restores  the  light,  but  it  produce-^  no  color. 
With  my  penknife  I  take  a  thin  splinter  from 
this  crystal  and  place  it  between  the  prism;-  { 
its  image  on  the  screen  glows  with  the  richest 
colors.  Turning  the  prism  in  front,  these 
colors  gradually  fade,  disappear,  but  by  con- 
tinuing the  rotation  until  the  vibrating  sec- 
tions of  the  prisms  are  parallel,  vivid  colors 
again  appear,  but  these  colors  are  comple- 
mentary to  the  former  ones. 

Some  patches  of  the  splinter  appear  of  one 
color,  some  of  another.  These  differences 
ar.:  due  to  the  different  thicknesses  of  the. 
film.  If  the  thickness  be  uniform,  the  color 
is  uniform.  Here,  for  instance,  is  a  stellar 
shape,  every  lozenge  of  the  star  being  a  film 
of  gypsum  of  uniform  thickness.  Each 
lozenge,  you  observe,  shows  a  brilliant  uni- 
fcrm  color.  It  is  easy,  by  shaping  our  films 
so  as  to  represent  flowers  or  other  objects, 
to  exhibit  such  objects  in  colors  unattainable 
by  art.  Here,  for  example,  is  a  specimen  of 


heart's-ease,  the  colors  of  which  you  might 
safely  defy  the  artist  to  reproduce.  By  turn- 
ing the  front  Nicol  ninety  degrees  round,  we 
pass  through  a  colorless  phase  to  a  series  of 
colors  complementary  to  the  foroier  ones. 
Here,  for  example,  is  a  rose  tree  with  red 
flowers  and  green  leaves;  turning  the  prism 
ninety  degrees  round,  we  obtain  a  green 
flower  and  red  leaves.  All  these  wonderful 
chromatic  effects  have  definite  mechanical 
causes  in  the  motions  of  the  ether.  The 
principle  of  interference,  duly  applied  and 
interpreted,  explains  them  all. 

By  this  time  you  have  learned  that  the 
word  "  light"  may  be  used  in  two  different 
senses  ;  it  may  mean  the  impression  made 
upon  consciousness,  or  it  may  mean  the  phys- 
ical agent  which  makes  the  impression.  It 
is  with  the  agent  that  we  have  to  occupy  our- 
selves at  present.  That  agent  is  the  motion 
of  a  substance  which  fills  ail  space,  and  sur- 
rounds the  atoms  and  molecules  of  bodies. 
To  this  interstellar  and  interatomic  medium 
definite  mechanical  properties  are  ascribed, 
and  we  deal  with  it  as  a  body  possessed  of 
these  p  operties.  In  mechanics  we  have  the 
composition  and  resolution  of  forces,  and  of 
motions,  extending  to  the  composition  and 
resolu.ion  of  vibrations.  We  treat  the  lumi- 
niferous  ether  on  mechanical  principles,  and 
from  the  composition,  resolution,  and  inter- 
ference of  its  vibrations,  we  deduc;  all  the 
phenomena  displayed  by  crystals  in  polarized 
light. 

Let  us  take,  as  an  example,  the  crystal  of 
tourmaline,  with  which  we  are  now  so  famil- 
iar. Let  a  vibration  cross  this  erysta*  oblique 
to  its  axis  ;  \vj  have  seen  by  experiment  tnat 
a  portion  of  the  light  will  pan  through. 
How  much,  we  determine  in  this  way :  Draw 
a  straight  line  representing  the  intensity  of 
the  vibration  before  it  reaches  the  tourmaline, 
and  from  the  two  ends  of  this  line  draw  two 
perpendiculars  to  the  axis  of  the  crystal ;  the 
distance  between  the  feet  of  these  two  per- 
pendiculars will  represent  the  intensity  01  Lae 
transmitted  vibration. 

Follow  me  now  while  I  endeavor  to  make 
clear  to  you  what  occurs  when  a  film  of 
gypsum  is  placed  between  the  JNicol  pri>ms. 
But,  at  the  outset,  let  us  cstabhsn  still 
further  the  analogy  between  the  action  of  the 
prisms  and  that  ot  two  plates  of  tourmaline, 
The  plates  are  now  crossed,  and  you  see  that 
by  turning  the  film  round,  it  may  be  placed 
in  a  position  where  i'  has  no  povver  to  -abolish 
the  darkness.  Why  is  this?  The  answer  is 
that  \<\  the  gypsum  there  are  two  directions, 
at  light  angles  to  each  other,  which,  the  waves 
of  light  are  constrained  to  follow,  ;:nd  that 
now  one  of  these  directions  is  parallel  to  one 
of  the  axes  of  the  tourmaline,  and  the  oiher 
parallel  to  the  other  axis.  When  this  is  the 
case,  the  film  exercises  no  sensible  action 
upon  the  light.  But  now  I  turn  the  film  so 
as  to  render  its  direction  of  vibration  oblique 
to  the  axes  ;  then  you  see  it  has  the  power, 


SIX  LECTURES  ON  LIGHT. 


demonstrated  in  tneiast  lecture,  of  restoring 
tne  light. 

Let  us  now  mount  our  Nicol  prisms  and 
cross  them  as  we  crossed  the  tourmalines. 
Introducing  our  film  of  gypsum  between  them 
you  notice  tnat  in  one  particular  position  the 
film  has  no  power  whatever  over  the  field  of 
view.  But,  when  the  film  is  turned  a  little 
way  round,  the  light  passes.  We  have  now 
to  understand  the  mechanism  by  which  this 
is  effected. 


Firstly,  then,  we  have  this  first  prism  which 
receives  the  light  emergent  from  the  electric 
(amp,  and  which  is  called  the  polarizer.  Then 
we  have  the  p  ate  of  gypsum,  placed  at  S 
(Fig.  17),  and  then  the  prism  in  front,  which 
is  called  the  analyzer.  On  its  emergence 
from  the  first  prism,  the  light  is  polarized  ; 
and  in  the  particular  case  now  before  us,  its 
vibrations  are  executed  in  an  horizontal  plane. 
The  two  directions  of  vibration  of  ;  he  gypsum, 
placed  at  S,  are  now  oblique  to  the  horizon. 
Draw  a  rectangular  cross  upon  paper  to  rep- 
resent the  two  directions  of  vibration  within 
the  gypsum.  Draw  an  oblique  line  to  repre- 
sent the  intensity  of  the  vibration  when  it 
reaches  the  gvpsum.  Let  fall  from  the  two 
ends  of  this  line  two  perpendiculars  on  each 
of  the  arms  of  the  cross  ;  then  the  distances 
between  the  feet  of  these  perpendiculars  rep- 
resent the  intensities  of  two  rectangular  vi- 
brations which  are  the  equivalents  of  the  first 
single  vibration.  Thus  the  polarized  ray, 
when  it  enters  the  gypsum,  is  resolved  into 
two  others,  vibrating  at  right  angles  to  each 
other. 

Now,  in  one  of  those  directions  of  vibnition 
the  ether  is  more  sluggish  than  in  the  other  ; 
and,  as  a  consequence,  the  waves  that  follow 
this  direction  are  more  retarded  than  the 
others.  The  waves  of  both  systems,  in  fact, 
are  shortened  when  they  enter  the  gypsum, 
but  the  one  system  is  more  shortened  than  the 


other.  You  can  readily  imagine  that  in  this 
way  the  one  system  of  waves  may  get  half  a 
wave-length,  or  indeed  any  number  of  halt 
wave-lengths,  in  advance  of  the  other.  The 
possibility  of  interference  here  flashes  upon 
the  mind.  A  little  consideration,  however, 
renders  it  evident  that,  as  long  as  the  vibra- 
tions are  executed  at  right  angles  to  each 
other,  they  cannot  quench  each  other,  no 
matter  what  the  retardation  may  be.  This 
brings  us  at  once  to  the  part  played  by  the 
analyzer.  Its  sole  function  is  to  recompound 
i  he  two  vibrations  emergent  from  the  gypsum. 
It  reduces  them  to  a  single  plane,  where,  if 
one  of  them  be  retarded  by  the  proper 
amount,  extinction  can  occur.  But  here,  as 
in  the  case  of  thin  films,  the  different  lengths 
of  the  waves  of  light  come  into  play.  Red 
will  require  a  greater  thickness  to  produce  the 
retardation  necessary  for  extinction  than  blue; 
consequently,  when  the  longer  waves  have 
been  withdrawn  by  interference,  the  shorter 
ones  remain  and  confer  their  colors  on  the 
film  of  gypsum.  Conversely,  when  the 
shorter  waves  have  been  withdrawn,  the 
thickness  is  such  that  the  longer  waves  re- 
main. An  elementary  consideration  suffices 
to  show  that,  when  the  directions  of  vibration 
of  prisms  and  gypsum  enclose  an  angle  of 
forty-five  degrees,  the  colors  are  at  their 
maximum  brilliancy.  When  the  film  is 
turned  from  this  direction,  the  colors  gradu- 
ally fade,  until,  at  the  point  where  the  direc- 
tions are  parallel,  they  disappear  altogether. 

A  knowledge  of  these  phenomena  ':*>  best 
obtained  by  means  of  a  model  of  wood  or 
aasteboard  representing  the  plate  of  gypsum, 
ts  planes  of  vibration,  and  also  those  of  the 
Dolarizer  and  analyzer.  On  these  planes  the 
waves  may  be  drawn,  showing  the  resolution 
of  the  first  polarized  ray  into  two  others,  and 
hen  the  reduction  of  tne  two  vibrations  to  a 
common  plane.  Following  out  ligidly  the 
nte/action  of  the  two  systems  of  waves,  we 
are  taught  by  such  a  model  that  a'l  the  phe- 
nomena of  color,  obtained,  when  the  planes  of 
vibration  of  the  two  Nicols  are  parallel,  are 
displaced  by  the  complementary  phenomena 
when  the  Nicols  are  perpendicular  to  each 
other. 

In  considering  the  next  point,  for  the  sake 
of  simplicity,  we  will  operate  with  monochro- 
matic light — with  red  light,  for  example. 
Supposing  that  a  certain  thickness  of  the  gyp- 
sum produces  a  retardation  of  half  a  wave- 
length, twice  this  thickness  will  produce  a 
retaidation  of  two  half  wave-lengths;  three 
times  this  thickness  a  retardation  of  three 
half  wave-lengths,  and  so  on.  Now,  when 
the  Nicols  are  parallel,  the  retardation  of 
half  a  wave-length,  cr  of  any  odd  number  D£ 
half  wave-lengths,  produces  extinction;  at  ail 
thicknesses,  on  the  other  hand,  which  corre- 
spond to  a  retardation  of  an  even  number  of 
half  wave-lengths,  the  two  beams  support  each 
other,  when  they  are  brought  to  a  common 
plane  by  the  analyzer.  S-pposing,  then, 


30 


SIX  LECTURES  ON  LIGHT. 


that  we  take  a  plate  of  a  wedge-form,  which 
grows  gradually  thicker  from  edge  to  back, 
we  ought  to  expect  in  red  light  a  series  of 
recurrent  bands  of  light  and  darkness  ;  the 
dark  bands  occurring  at  thicknesses  which 
produce  retardations  of  one,  three,  five,  etc., 
half  wave-lengths,  uhile  the  light  bands  occur 
between  the  dark  ones.  Expeiiment  proves 
the  wedge-shaped  crystal  to  show  these  bands 
but  they  are  far  better  shown  by  this  circular 
film,  which  is  so  worked  as  to  be  thinnest  at 
the  centre,  gradually  increasing  in  thickness 
from  the  centre  outwards.  These  splendid 
rings  of  light  and  darkness  are  thus  produced. 
When,  instead  of  employing  red  light,  we 
employ  blue,  the  rings  are  also  seen  ;  but  as 
they  occur  at  thinner  portions  of  the  film, 
they  are  smaller  than  the  rings  obtained  with 
the  red  light.  The  consequence  of  employ- 
ing %uhite  light  may  now  be  inferred  :  inas- 
much as  the  red  and  the  blue  fall  in  different 
places,  we  have  iris-colored  rings  produced  by 
the  white  light. 

Some  of  the  cVomatic  effects  of  irregular 
crystallization  are  beautiful  in  the  extreme. 
Could  I  introduce  between  our  Nicols  a  pane 
of  glass  covered  by  those  frost-ferns  which 
the  cold  weather  renders  now  so  frequent, 
rich  colors  would  be  the  result.  The  beauti- 
ful effects  of  irregular  crystallization  on  glass 
plates,  row  presented  to  you,  illustrate  what 
you  might  e.\pect  from  the  irosted  window- 
pane.  And  not  only  do  crystalline  bodies 
act  thus  upon  light,  but  almost  all  bodies  that 
possess  a  definite  structure  do  the  same.  As 
a  general  rule,  organic  bodies  act  in  this  way; 
for  their  architecture  implies  an  arrangement 
of  the  ether  which  involves  double  refraction. 
A  film  of  horn,  or  the  section  of  a  shell,  for 
example,  yields  very  beautiful  colors  in  polar- 
ized light.  In  a  tree,  the  ether  certainly  pos- 
sesses different  degrees  of  elasticity  along  and 
across  the  fibre;  and,  were  wood  transparent, 
this  peculiarity  of  molecular  structure  would 
infallibly  reveal  itself  by  chromatic  phe- 
nomena like  those  that  you  have  seen.  But 
not  only  do  todies  built  permanently  by 
Nature  behave  in  this  way,  but  it  is  possible, 
as  shown  by  Brewster,  to  confer,  by  strain  or 
by  pressure,  a  temporary  double-refracting 
structure  upon  non-crystalline  bodies,  such  as 
common  glass. 

When  I  place  this  bar  of  wood  across  my 
knee  and  seek  to  break  it,  what  is  the 
mechanical  condition  of  the  bar  ?  It  bends, 
and  its  convex  surface  is  strained  longitudi- 
nally; its  concave  surface,  that  next  my  knee, 
is  longitudinally  pressed.  Both  in  the 
strained  portion  and  in  the  pressed  portion 
the  ether  is  thrown  into  a  condition  which 
would  rend  r  the  wood,  were  it  transparent, 
double  refracting.  Let  us  repeat  the  experi- 
ment with  a  bar  of  glass.  Between  the 
crossed  Nicols  I  introduce  such  a  bar.  By 
the  dim  residue  of  light  lingering  upon  the 
screen,  you  see  the  image  of  the  glass,  but  it 
has  no  effect  upon  the  light.  I  simply  bend 


the  gla^s  bar  with  my  finger  and  thumb, 
keeping  its  length  oblique  to  the  directions  of 
vibration  in  the  Nicols.  Instantly  light 
flashes  out  upon  the  screen.  The  two  sides 
of  the  bar  are  illuminated,  the  ed.jes  most, 
for  here  the  strain  and  pressure  are  greatest. 
In  passing  from  strain  to  pressure,  we  cross  a 
portion  of  the  glass  where  neither  is  exerted. 
This  is  the  so-called  neutral  axis  of  the  bar 
of  glass,  and  along  it  you  see  a  dark  band, 
indicating  that  the  j;lass  along  this  axis  exer- 
cises no  .  ction  upon  the  light.  By  employ- 
ing the  force  of  a  press,  instead  of  the  lorce 
of  my  finger  and  thumb,  the  brilliancy  of  the 
light  is  greatly  augmented. 

Again,  I  have  here  a  square  of  glass  which 
can  be  inserted  into  a  press  of  another  kind. 
Introducing  the  square  between  the  prisms, 
its  neutrality  is  declared  ;  but  it  can  hardly 
be  held  sufficiently  loosely  to  prevent  its 
action  from  manifesting  itself.  Already, 
though  the  pressure  is  infinitesimal,  you  see 
spots  of  light  at  the  points  where  the  press  is 
in  contact  with  the  glass.  I  now  turn  this 
screw.  Instantly  the  image  of  the  square  of 
glass  flashes  out  upon  the  screen.  You  see 
luminous  spaces  separated  from  each  other 
by  dark  bands.  Every  pair  of  adjacent 
luminous  spaces  is  in  opposite  mechanical 
conditions.  On  one  side  of  the  dark  band 
we  have  strain,  on  the  other  side  pressure  ; 
while  the  dark  band  marks  the  neutral  axis 
between  both.  I  now  tighten  the  vise,  and 
you  see  color ;  tighten  still  more,  and  the 
colors  appear  as  rich  as  those  presented  by 
crystals.  Releasing  the  vise,  the  colors 
suddenly  vanish  ;  tightening  suddenly,  they 
reappear.  From  the  colors  of  a  soap-bubble 
Newton  was  able  to  infer  the  thickness  of  the 
bubble,  thus  uniting  by  the  bond  of  thought 
apparently  incongruous  things.  From  the 
colors  here  presented  to  you,  the  magnitude 
of  the  pressure  employed  might  be  inferred. 
Indeed,  the  late  M.  Wertheim,  of  Paris,  in- 
vented an  instrument  for  the  determination 
of  strains  and  pressures  by  the  colors  of 
polarized  light,  which  exceeded  in  accuracy 
all  other  instruments  of  the  kind. 

You  know  that  bodies  are  expanded  by 
heat  and  contracted  by  cold.  If  the  heat  be 
applied  with  perfect  uniformity,  no  local 
strains  cr  pressures  come  into  play  ;  but,  if 
one  portion  of  a  solid  be  heated  and  others 
not,  the  expansion  of  the  heated  portion  intro- 
duces strains  and  pressures  which  reveal 
themselves  under  the  scrutiny  of  polarized 
ight.  When  a  square  of  common  window- 
jlass  is  placed  between  the  Nicols,  you  see 
ts  dim  outline,  but  it  exerts  no  action  on  the 
polarized  light.  Held  for  a  moment  over  the 
:lame  of  a  spirit-lamp,  on  reintroducing  it 
between  the  Nicols,  light  flashes  out  upon 
the  screen.  Here,  as  in  the  case  of  mechan- 
cal  action,  you  have  spaces  of  strain  divided 
3y  neutral  axes  from  spaces  of  pressure. 

Let  us  apply  the  heat  more  symmetrically. 
This  small  square   of  glass  is  perforated  at 


SIX  LECTURES  ON  LIGHT. 


the  centre,  and  into  the  orifice  a  bit  of  copper 
wire  is  intioduced.  Placing  the  square  be- 
tween the  prisms,  and  heating  the  copper, 
thft  heat  passes  by  conduction  along  the  wire 
to  the  glass,  through  which  it  spreads  from 
the  centre  outwards.  You  sse  a  dim  cross 
bounding  four  luminous  quadrants  growing 
up  and  becoming  gradually  black  by  compari- 
son with  the  adjacent  brightness.  And  as,  in 
the  case  of  pressure,  we  produced  colors,  so 
here  also,  by  the  proper  application  of  heat, 
gorgeous  chromatic  effects  may  be  produced, 
and  they  may  be  rendered  permanent  by  first 
heating  the  glass  sufficiently,  and  then  cool- 
ing it,  so  that  the  chilled  mass  shall  remain 
in  a  state  of  strain  and  pressure.  Two  or 
three  examples  will  illustrate  this  point.  The 
colors,  you  observe,  are  quite  as  rich  as  those 
obtained  in  the  case  of  crystals. 

And  now  we  have  to  push  these  considera- 
tions to  a  final  illustration.  Polarized  light 
may  be  turned  to  account  in  various  ways  as 
an  analyzer  of  molecular  condition.  A  strip 
of  glass  six  feet  long,  two  inches  wide,  and 
a  quarter  of  an  inch  thick,  is  held  at  the 
centre  between  my  finger  and  thumb.  I 
sweep  over  one  of  its  halves  a  wet  woolen 
rag  ;  you  hear  an  acute  sound,  due  to  the 
vibrations  of  the  glass.  What  is  the  condi- 
tion of  the  glass  while  the  sound  is  heard  ? 
This  .  its  two  halves  lengthen  and  shorten  in 
quick  succession.  Its  t\vo  ends,  therefore, 
aie  in  a  state  of  quick  vibration  ;  but  at  the 
centre  the  pulses  from  the  two  ends  alter- 
nately meet  and  retreat.  Between  their 
opposing  actions,  the  glass  at  the  centre  is 
kept  motionless  ;  but,  on  the  other  hand,  it 
is  alternately  strained  and  compressed.  The 
state  of  the  glass  may  be  illustrated  by  a  row 
of  spots  of  light,  as  the  propagation  of  a 
sonorous  pulse  was  illustrated  in  a  former 


FIG.  1 8. 

lecture.  By  a  simple  mechanical  contrivance 
the  spots  are  made  to  vibrate  to  and  fro. 
The  terminal  dots  have  the  largest  amplitude 


of  vibration,  while  those  at  the  centre  are 
alternately  crowded  together  and  torn 
asunder,  the  centre  one  not  moving  at  all. 
The  condition  of  the  sounding  strip  of  glass 
is  here  correctly  represented.  In  Fig.  18,  A 
B  represents  the  glass  rectangle  with  its 
centre  condensed  ;  while  A'  B'  represents  the 
same  rectangle  with  its  centra  rarefied. 

If  we  introduce  the  glass  s  s'  (Fig.  19)  be- 
tween the  crossed  Nicols,  taking  care  to  keep 
the  strip  oblique  to  the  direction  of  vibration 
of  the  Nicols,  and  sweep  our  wet  rubber  over 
the  glass,  this  may  bi  expected  to  occur  :  At 
every  moment  of  compression  the  light  will 
flash  through  ;  at  every  moment  of  strain  the 
light  will  also  ilash  through  ;  and  these  states 
of  strain  and  pressure  will  follow  each  other 
so  rapidiy  that  we  may  expect  a  permanent 
luminous  impression  to  be  made  upon  the 
eye.  By  pure  reasoning,  therefore,  we  reach 
the  conclusion  that  the  light  will  be  revived 
whenever  the  glass  is  sounded.  That  it  is  so, 
experiment  testifies  :  at  every  sweep  of  the 
rubber,  a  fine  luminous  disk  (o)  flashes  out 
upon  the  screen.  The  experiment  may  be 
varied  in  this  way :  Placing  in  fron  of  the 
polarizer  a  plate  of  unannealed  glass,  you 
have  those  beautiful  colored  rings,  intersected 
by  a  black  cross.  Every  sweep  of  the  rubber 
not  only  abolishes  the  rings,  but  introduces 
complementary  ones,  the  black  cross  bein^ 
for  the  moment  supplanted  by  a  white  one. 
This  is  a  modification  of  an  experiment 
which  we  owe  to  Biot.  His  apparatus,  how- 
ever, confined  the  observation  of  it  to  a  single 
person  at  a  time. 

But  we  have  to  follow  the  ether  still 
further.  Suspended  before  you  is  a  pendu- 
lum, which,  when  drawn  aside  and  then 
liberated,  oscillates  to  and  fro.  If  when  the 
pendulum  is  passing  the  middle  point  of  its 
excursion,  I  impart  a  shock  to  it  tending  to 
drive  it  at  right  angles  to  its  present  course, 
what  occurs?  The  two  impulses  compound 
themselves  to  a  vibration  oblique  in  direction 
to  the  former  one,  but  the  pendulum  oscil- 
lates in  a  plane.  But,  if  the  rectangular 
-hock  be  imparted  to  the  pendulum  whpn  it 
is  at  the  limit  of  its  swing,  then  the  com- 
pounding of  the  two  impulses  causes  the  sus- 
pended ball  to  describe  not  a  straight  line, 
but  an  ellipse ;  and,  if  the  shock  be  compe- 
tent of  itself  to  produce  a  vibration  of  the 
same  amplitude  as  the  first  one,  the  ellipse 
becomes  a  circle.  But  why  do  I  dwell  upon 
these  things  ?  Simply  to  make  known  to  you 
the  resemblance  of  these  gross  mechanical 
vibrations  to  the  vibrations  of  light.  1  hold 
in  my  hand  a  plate  of  quartz  cut  from  the 
crystal  perpendicular  to  its  axis.  This  crys- 
tal thus  cut  possesses  the  extraordinary  power 
of  twisting  the  plane  of  vibration  of  a.  polar- 
ized ray  to  an  extent  dependent  on  the  thick- 
ness of  the  crystal.  And  the  more  refrangi- 
ble the  light  the  greater  is  the  amount  of 
twisting,  so  that,  when  white  light  is  em- 
ployed, its  constituent  colors  are  thus  drawo 


SIX  LECTURES  ON  LIGHT. 


asunder.  Placing1  the  quartz  between  the 
polarizer  and  the  analyzer,  you  see  this 
splendid  color,  and,  turning  the  analyzer  in 
front,  from  right  to  left,  the  other  colors 
apoear  in  succession.  Specimens  of  quartz 
have  been  found  which  require  the  analyzer 
to  be  turned  from  left  to  right,  to  obtain  the 
same  succession  of  colors.  Crystals  of  the 
first  class  are  therefore  called  right-handed, 
ar.d,  of  the  second  class,  left  handed  crystals. 
With  profound  sagacity,  Fresne  ,  to  whose 
genius  \ve  mainly  owe  the  expansion  and 
final  triumph  of  the  undulatory  theory  of 
light,  reproduced  mentally  the  mechanism  of 
these  crystals,  and  showed  their  action  to  be 
due  to  the  circumstance  that,  in  them,  the 
v/aves  of  ether  so  act  upon  each  other  as  to 
produce  the  condition  represented  by  our 
rotating  pendulum.  Instead  of  being  plane 
polarized,  the  light  in  rock  crystal  is  circu- 
larly polarized.  Two  such  rays  transmitted 
along  the  axis  of  the  crystal,  and  rotating  in 


although  the  mixture  of  blue  and  yellow  pig- 
ments produces  green,  the  mixture  of  biuc 
and  yellow  lights  produces  white.  By  en- 
larging our  aperture,  the  two  images  pro- 
duced by  the  spar  are  caused  to  approach 
each  other,  and  finally  to  overlap.  The  one 
is  now  a  vivid  yellow,  the  other  a  vivid  blue* 
and  you  notice  that  where  the  colors  are 
superposed  we  have  a  pure  white.  (See  Fig. 
20,  where  N  is  the  nozzle  of  the  lamp,  Q  the 
quartz  plate,  L  a  lens,  and  B  the  birefracting 
spar.  The  two  images  overlap  at  O,  and 
produce  white  by  their  mixture.) 

This  brings  us  to  a  point  of  our  inquiries 
which,  though  not  capable  of  brilliant  illus- 
tration, is  nevertheless  so  likely  to  affect  pro- 
foundly the  future  course  of  scientific  thought 
that  I  am  unwilling  to  pass  it  over  without 
reference.  I  refer  to  the  experiment  which 
Faraday,  its  discoverer,  called  the  magnetiza- 
tion of  light.  The  arrangement  for  thL 
celebrated  experiment  is  now  before  you. 


FIG.  19. 


opposite  directions,  when  brouih':  to  inter- 
ference by  the  analyzer,  are  demonstrably 
competent  to  produce  the  observed  phe- 
nomena. 

1  now  abandon  the  analyzer,  and  put  in  its 
place  the  piece  of  Iceland  spar  with  which 
we  have  already  illustrated  double  refraction. 
The  two  images  of  the  carbon-points  are 
now  before  you.  Introducing  a  plate  of 
quartz  between  the  polarizer  and  the  spar, 
the  two  images  glow  with  complementary 
colors.  Emplo)ing  the  image  of  an  aperture 
instead  of  that  of  the  carbon-points,  we  have 
two  complementary  colored  circles.  As  the 
analyzer  is  caused  to  rotate,  the  colors  pass 
through  various  changes  ;  but  they  are  al- 
ways complementary  to  each  other.  If  the 
one  be  red,  the  other  will  be  green  ;  if  the 
one  be  yellow,  the  other  will  be  blue.  Here 
we  have  it  in  our  power  to  demonstrate  afresh  j 
a  statement  made  in  a  former  lecture,  that,  i 


We  have  first  our  electric  lamp,  then  n  Nicot 
prism,  to  polarize  the  beam  emergeni  from 
the  lamp  ;  then  an  electro-magnet,  then  a 
second  Nicol  prism,  and  finally  our  screen. 
At  the  present  moment  the  prisms  are 
crossed,  and  the  screen  is  dark.  I  place 
from  pole  to  pole  of  the  electro  magnet  a 
cylinder  of  a  peculiar  kind  of  glass,  first 
made  by  Faraday,  and  called  Faraday's  heavy 
glass.  Through  this  glass  the  b^am  from 
the  polarizer  now  passes,  being  intercepted 
by  the  Nicol  in  front.  I  now  excite  the 
magnet,  and  instantly  light  appears  upon  the 
screen.  On  examination,  we  find  that,  by 
the  action  of  the  magnet  upon  the  ether  con- 
tained within  the  heavy  glass,  the  plane  01 
vibration  is  caused  to  rotate,  thus  enabling 
the  light  to  get  through  the  anaFyzer. 

The  two  classes  into  which  quartz-crystals 
are  divided  have  been  already  mentioned. 
In  my  hand  I  hold  a  compound  piate,  ons- 


SIX  LECTURES  ON  LIGHT. 


half  of  it  taken  from  a  right-handed  and  the 
other  from  a  left-handed  crystal.  Placing 
the  plate  in  front  of  the  polarizer,  we  turn 
one  of  the  Nicols  until  the  two  halves  of  the 
plate  show  a  common  puce  color.  This 
yields  an  exceedingly  sensitive  means  of  ren- 
dering the  action  of  a  magnet  upon  light 
By  turning  either  the  polarizer  or 


the  analyzer  through  the  smallest  angle,  tne 
uniformity  of  the  color  disappears,  and  the 
two  halves  of  the  quartz  show  different  colors. 
The  magnet  also  produces  this  effect.  The 
puce-coicred  circle  is  now  before  you  on  the 
screen.  (See  Fig.  21  for  the  arrangement  of 
the  experiment.  N  is  the  nozzle  of  the  lamp, 
H  the  first  Nico),  Q  the  biquartz  plate,  L  a 
lens,  M  the  electro-magnet,  and  P  the  second 
Nicol.)  Exciting  the  magnet,  one  half  of 
the  image  becomes  suddenly  red,  the  other 
half  green.  Interrupting  the  current,  the 
two  colors  fade  away,  and  the  primitive  puce 
is  restored.  The  action,  moreover,  depends 
upon  the  polarity  of  the  magnet,  or,  in  other 
words,  on  the  direction  of  the  current  which 
surrounds  the  magnet.  Reversing  the  cur- 
rent, the  red  and  green  reappear,  but  they 
have  changed  places.  The  red  was  for- 
merly to  the  right,  and  the  green  to  the  left ; 
the  green  is  now  to  the  right,  and  the  red  to 
the  left.  With  the  most  exquisite  ingenuity, 
Faraday  analyzed  all  those  actions  ar.d  stated 
their  laws.  This  experiment,  however,  long 
remained  rather  as  a  scientific  curiosity  than  as 
a  fruitful  germ.  That  it  would  bear  fruit  of 
the  highest  ;mportance,  Faraday  felt  pro- 
foundly convinced,  and  recent  researches  are 
on  the  way  to  verify  his  conviction. 

A  few  words  more  are  necessary  to  com- 
plete our  knowledge  ot  the  wonderful  inter- 
action between  ponderable  molecules  and  the 
ether  interfused  among  them.  Symmetry  of 


molecular  arrangement  implies  symmetry  on 
the  part  of  the  ether  ;  atomic  dissymmetry, 
on  the  other  hand,  involves  the  dissymmetry 
of  the  ether,  and,  as  a  consequence,  double 
refraction.  In  a  certain  class  of  crystals  the 
structure  is  homogeneous,  and  such  crystals 
produce  no  double  refraction.  In  certain 


other  crystals  the  moJr  jles  are  ranged  sym- 
metrically around  ?  Certain  line,  and  not 
around  others.  A'ouglhe  former,  therefore, 
the  ray  is  undivided,  while  along  ail  the 
others  we  have  double  refraction.  Ice  is  a 
familiar  example  ;  it  is  built  with  perfect 
symmetry  around  the  perpendiculars  to  the 
planes  of  freezing,  and  a  ray  sent  through 
ice  in  this  direction  is  not  doubly  refracted  ; 
whereas,  in  all  other  directions,  it  is.  Ice- 
land spar  is  another  example  of  the  same 
kind  :  its  molecules  are  built  symmetrically 
round  the  line  uniting  the  two  blunt  angles 
of  the  rhomb.  In  this  direction  a  ray  suffers 
no  double  refraction,  in  all  others  it  does. 
This  direction  of  double  refraction  is  called 
the  optic  axis  of  the  crystal. 

Hence,  if  a  plate  be  cut  from  a  crystal  of 
Iceland  spar  perpendicular  to  the  3x:s,  a." 
rays  sent  across  this  plate  in  the  dirction  c 
the  axis  will  produce  but  cne  image.  Bu* 
the  moment  we  deviate  from  the  parallelism 
with  the  axis,  double  refraction  sets  in.  If, 
therefore,  a  beam  that  has  been  rendered 
conical  by  a  converging  lens  be  sent  through 
the  spar  so  that  the  central  ray  of  the  cone 
passes  along  the  axis,  this  ray  only  will  es- 
cape double  refraction.  Each  of  the  others 
will  be  divided  into  an  ordinary  and  extraor- 
dinary ray,  ihe  one  moving  more  slowly 
through  the  crystal  than  the  other  ;  the  one, 
therefore,  retarded  with  reference  to  the 
other.  Here,  then,  we  have  the  conditions 


SIX  LECTURES  ON  LIGHT. 


for  interference,  when  the  waves  are  reduced 
by  the  analyzer  to  a  common  plane.  A 
highly  beautiful  and  important  source  of 
chromatic  phenomena  is  thus  revealed. 
Placing  the  plate  of  spar  between  the  crossed 
prisms  we  have  upon  the  screen  a  beautiful 
system  of  iris  rings  surrounding  the  end  of 
the  optic  axis,  the  circular  bands  of  color 
being  intersected  by  a  black  cross.  The 
arms  ^f  th:s  cross  are  parallel  to  the  two 
directions  of  vibration  in  the  polarizer  and 
analyzer.  It  is  easy  to  see  that  those  rays 
whose  planes  of  vibration  within  the  spar 
coincide  with  the  plane  of  vibration  of  either 
prism,  cannot  get  through  both.  This  com- 
plete interception  produces  the  arms  of  the 
cross.  With  mono-chromatic  light  the  rings 
would  be  simply  bright  and  black — the 
bright  rings  occurring  at  those  thicknesses  of 
the  spar  which  cause  the  rays  to  conspire  ; 
the  black  rings  at  those  thicknesses  which 
cause  them  to  quench  each  other.  Here, 
however,  as  elsewhere,  the  different  lengths 
of  the  light-waves  give  rise  to  iris-colors 
when  white  light  is  employed. 

Besides  the  regular  crystals  which  produce 
double  refraction  in  no  direction,  and  the 
nniaxal  crystals  which  produce  it  in  all  direc- 
tions but  one,  Brewster  discovered  that  in  a 
large  class  of  crystals  there  are  two  directions 
in  which  double  refraction  does  not  take 
place.  These  are  called  biaxal  crystals. 
When  plates  ot  these  crystals,  suitably  cut, 
,  are  placed  between  the  polarizer  and  analyzer, 
Ihe  axes  are  seen  su  -ounded,  not  by  circles, 
but  by  curves  of  ano  her  order  and  of  a  per- 
fectly definite  mathem*  :cal  character.  Each 
band,  as  proved  experimentally  by  Herschel, 
forms  a  lemniscata ;  but  the  experimental 
proof  was  here,  as  in  numberless  other  cases, 
preceded  by  the  deduction  which  showed  that, 
according  to  the  undulatory  theory,  the  bands 
must  possess  this  special  character. 

I  have  taken  this  somewhat  wide  range 
over  polarization  itselt  and  over  the  phenom- 
ena exhibited  by  crystals  in  polarized  light, 
in  order  to  give  you  some  notion  of  the  firm- 
ness a. id  completeness  of  the  theory  which 
grasps  them  all.  Starting  from  the  single 
assumption  of  transverse  undulations,  we 
first  of  all  determine  the  wave-lengths,  and 
find  all  the  phenomena  of  color  dependent 
on  this  element.  The  wave-lengths  may  be 
determined  in  many  independent  ways,  and, 
when  the  lengths  so  determined  are  compared 
together,  the  strictest  agreement  is  found  to 
exist  between  them.  We  follow  the  ether 
into  the  most  complicated  cases  of  interac- 
tion between  it  and  ordinary  matter,  "the 
theory  is  equal  to  them  all.  It  makes  not  a 
single  new  hypothesis  ;  but  out  of  its  original 
stock  of  principles  it  educes  the  counterparts 
of  all  that  observation  shows.  It  accounts 
for,  explains,  simplifies  the  most  entangled 
cases  ;  corrects  known  laws  and  facts  ;  pre- 
dicts an  1  discloses  unknown  ones  ;  becomes 
the  guide  of  its  former  teacher  Observation  ; 


and,  enlightened  by  mechanical  conceptions, 
acquires  an  insight  which  pierces  through 
shape  and  color  to  force  and  cause.  "* 

But,  while  I  have  thus  endeavored  to  illus- 
trate before  you  the  power  of  the  undulatory 
theory  as  a  solver  of  all  the  difficulties  of 
optics,  do  I  therefore  wish  you  to  close  your 
eyes  to  any  evidence  that  may  arise  against 
it  ?  By  no  means.  You  may  urge,  and 
justly  urge,  that  a  hundred  years  ago  another 
theory  was  held  by  the  most  eminent  men, 
and  that,  as  the  theory  then  held  had  to 
yield,  the  undulatory  theory  may  have  to 
yield  also.  This  is  perfectly  logical ;  but  let 
us  understand  the  precise  value  of  the  argu- 
ment. In  similar  language  a  person  in  the 
time  of  Newton,  or  even  in  our  time,  might 
reason  thus:  "  Hipparchus  and  Ptolemy, 
and  numbers  of  great  men  after  them,  be- 
lieved that  the  earth  was  the  centre  of  the 
solar  system.  But  this  deep-set  theoretic 
notion  had  to  give  way,  and  the  theory  of 
gravitation  may,  in  its  turn,  have  to  give 
way  also."  This  is  just  as  logical  as  the  first 
argument.  Wherein  consists  the  strength  of 
the  theory  of  gravitation  ?  Solely  in  its  com- 
petence to  account  for  all  the  phenomena  of  the 
solar  system.  Wherein  consists  the  strength 
of  the  theory  of  undulation?  Solely  in  its 
competence  to  disentangle  and  explain  phe- 
tnomena  a  hundred-fold  more  complex  than 
those  of  the  solar  system.  Be  as  skeptical, 
if  you  like,  regarding  the  undulatory  theory  ; 
but  if  your  skepticism  be  philosophical,  it 
will  wrap  the  theory  of  gravitation  in  the 
same  or  greater  doubt,  f 


LECTURE  V. 

Range  of  vision  incommensurate  with  Range  of  Radi- 
ation: The  Ultra- Violet  Rays:  Fluorescence: 
Rendering  Invisible  Rays  visible:  Vision  not  the 
only  Sense  appealed  to  by  the  Solar  and  Electric 
Beam  :  Heat  of  Beam  :  Combustion  by  Total  Beam 
at  the  Foci  of  Mirrors  and  Lenses:  Combustion 
through  Ice-Lens:  Ignition  of  Diamond:  Search 
for  the  Rays  here  effective  :  Sir  »Villiain  Herschel's 
Discovery  of  Dark  Solar  Rays:  Invisible  Rays  the 
Basis  of  ttie  Visible  :  Detachment  by  a  Ray-Filter 
of  the  Invisible  Rays  from  the  Visible;  Combustion 
at  Dark  Foci:  Conversion  of  Heat- Rays  into 
Light-Rays:  Calorescence :  Part  played  in  Nature 
by  Dark  Rays:  Identity  of  Light  and  Radiant 
Heat:  Invisible  Images:  Reflection,  Refraction, 
Plane  Polarization,  Depolariz.ition.  Circular  Polar- 
ization, Double  Refraction,  and  Magnetization  of 
Radiant  Heat. 

THE  first  question  that  we  have  to  con- 
sider to-night  is  this  :  Is  the  eye,  as  an  organ 
of  vision,  commensurate  with  the  wnole 
range  of  solar  radiation — is  it  capable  of  re- 
ceiving visual  impressions  from  all  the  rays 
emitted  by  the  sun?  The  answer  is  nega- 
tive. If  we  allowed  ourselves  to  accept  for  a 


*  Whewell. 

t  The  only  essay  known  to  me  on  the  Undulatory 
Theory,  from  the  pen  of  an  American  writer,  is  an 
excellent  one  by  President  Barnard,  published  in  ths 
Smithsonian  Report  for  1862. 


SIX  LECTURES  ON  LIGHT. 


35 


moment  that  notion  of  gradual  growth, 
amelioration,  and  ascension,  implied  by  the 
term  evolution,  we  might  fairly  conclude  that 
there  a*-e  stores  of  visual  impressions  awaiting 
man  far  greater  than  those  of  which  he  is 
now  in  possession.  For  example,  here  be- 
yond the  extreme  violet  of  the  spectrum  there 
is  a  vast  efflux  of  rays  which  are  totally  use- 
less as  regards  our  present  powers  of  vision. 
But  these  ultra-violet  waves,  though  incom- 
petent to  awaken  the  optic  nerve,  can  so 
shake  the  molecules  of  certain  compound  sub- 
stances as  to  effect  their  decomposition.  The 
grandest  example  of  the  chemical  action  of 
light,  with  which  my  friend  Dr.  Draper  has 
so  indissolubly  associated  his  name,  is  that  of 
the  decomposition  of  carbonic  acid  in  the 
leaves  of  plants.  All  photography  is  founded 
on  such  actions.  There  are  substances  on 
which  the  ultra-violet  waves  exert  a  special 
decomposing  power  ;  and,  by  permitting  the 
invisible  spectrum  to  fall  upon  surfaces  pre- 
pared with  such  substances,  we  leveal  both 
the  existence  and  the  extent  of  the  ultra- 
violet spectrum. 

This  mode  of  exhibiting  the  action  of  the 
ultra-violet  rays  has  been  long  known  ;  in- 
deed, Thomas  Young  photographed  the  ultra- 
violet rings  of  Newton.  We  have  now  to 
demonstrate  their  presence  in  another  way. 
As  a  general  rule,  bodies  transmit  light  or 
absorb  it,  but  there  is  a  third  case  in  which  the 
light  falling  upon  the  body  is  neither  trans- 
mitted nor  absorbed,  but  converted  into  light  of 
another  kind.  Professor  Stokes,  the  occupant 
of  the  Chair  of  Newton  in  the  University  of 
Cambridge,  one  of  those  original  workers 
who,  though  not  widely  known  beyond 
scientific  circles,  realiy  constitute  the  core  of 
science,  has  demonstrated  this  change  of  one 
kind  of  light  into  another,  and  has  pushed 
his  experiments  so  far  as  to  render  the  invisi- 
ble rays  visible. 

A  long  list  of  substances  examined  by 
Stokes  when  excited  by  the  invisible  ultra- 
violet waves,  have  been  proved  to  emit  light. 
You  know  the  rate  of  vibration  corresponding 
to  the  extreme  violet  of  the  spectrum  ;  you 
are  aware  that,  to  produce  the  impression  of 
this  color,  the  retina  is  struck  789  millions  of 
millions  of  times  in  a  second.  At  this  point, 
the  retina  ceases  to  be  useful  as  an  organ  of 
vision,  for,  though  struck  by  waves  of  more 
rapid  recurrence,  they  are  incompetent  to 
awaken  the  sensation  of  light.  But,  when 
such  non-visual  waves  are  caused  to  impinge 
upon  the  molecules  of  certain  substances — 
on  those  of  sulphate  of  quinine,  for  example 
— they  compel  those  molecules,  or  their  con- 
stituent atoms,  to  vibrate  ;  and  the  peculiar- 
ity is,  that  the  vibrations  thus  set  up  are  of 
slower  period  than  those  of  the  exciting 
waves.  By  this  lowering  of  tr.e  rate  of  vi- 
bration through  the  intermediation  of  the  sul- 
phate of  quinine,  the  invisible  rays  are  ren- 
dered visible.  Here  we  have  our  spectrum, 
and  beyond  the  violet  I  place  this  prepared 


paper.  The  spectrum  is  immediately  elonga- 
ted by  the  generation  of  a  new  light  beyond 
the  extreme  violet.  President  Morton  has 
recently  succeeded  in  discovering  a  substance 
of  great  sensibility  which  he  has  named 
Thalloie,  and  he  has  been  good  enough  to 
favor  me  with  some  paper  saturated  with  a 
solution  of  this  substance.  It  causes  a  very 
striking  enlongation  of  the  spectrum,  the 
new  light  generated  being  of  peculiar  bril- 
liancy. To  this  change  of  the  rays  from  a 
higher  to  a  lower  refrangibility,  Stokes  has 
given  the  name  of  Fluorescence. 

By  means  of  a  deeply-colored  violet  glass, 
we  cut  off  almost  the  whole  of  the  light  of 
our  electric  beam  ;  but  this  glass  is  peculiarly 
transparent  to  the  violet  and  ultra-violet 
rays.  The  violet  beam  now  crosses  a  large 
jar  filled  with  water.  Into  it  I  pour  a  solu- 
tion of  sulphate  of  quinine :  opaque  clouds, 
to  all  appearance,  instantly  tumble  down- 
wards.  But  these  are  not  clouds  :  there  is 
nothing  precipitated  here  :  the  observed  ac- 
tion is  a  >  action  of  molecules,  not  of  particles. 
The  medium  before  you  is  not  a  turbid  me- 
dium, for,  when  you  look  through  it  at  a 
luminious  surface,  it  is  perfectly  clear.  If  we 
paint  upon  a  piece  of  paper  a  flower  or  a 
bouquet  with  the  sulphate  of  quinine,  and  ex- 
pose it  to  the  full  beam,  scarcely  anything  is 
seen.  But  on  interposing  the  violet  glass, 
the  design  instantly  flashes  forth  in  strong 
contrast  with  the  deep  surrounding  violet. 
Here  is  such  a  design  prepared  for  me  by 
President  Morton  with  his  thallene  :  placed 
in  the  violet  light  it  exhibits  a  peculiarly 
vivid  and  beautiful  fluorescence.  From  the 
experiments  of  Dr.  Bence  Jones,  it  would 
seem  that  there  is  some  substance  in  the  hu- 
man body  resembling  the  sulphate  of  quinine, 
which  causes  ail  the  tissues  of  the  body  to  be 
more  or  less  fluorescent.  The  crystalline 
lens  of  the  eye  exhibits  the  effect  in  a  very 
striking  manner.  When  I  plunge  my  eye 
into  this  violet  beam,  I  am  conscious  of  a 
whitish-blue  shimmer  filling  the  space  before 
me.  This  is  caused  by  fluorescent  light  gen- 
erated in  the  eye  itself  ;  looked  at  from  with- 
out, the  crystalline  lens  at  the  same  time 
gleams  vividly. 

But  the  waves  from  our  incandescent  car- 
bon-points appeal  to  another  s^nse  than  that 
of  vision.  They  not  only  produce  light  as  a 
sensation;  they  also  produce  heat.  The  mag- 
nified image  of  the  carbon-points  is  now  upon* 
the  screen,  and  with  a  suitable  instrument  the 
heating  power  of  that  instrument  might  be 
demonstrated.  Here,  however,  the  heat  is 

pread  over  too  large  an  area  to  be  intense. 
By  pushing  out  the  lens  and  causing  a  mova- 
ble screen  to  approach  our  lamp,  the  imige 
becomes  smaller  and  smaller  :  the  rays  be- 
come more  concentrated,  until  finally  they 
are  able  to  pierce  black  paper  with  a  burning 

ing.  Rendering  the  beam  parallel,  and  re- 
ceiving it  upon  a  concave  mirror,  the  rays  are 


36 


SiX  LECTURES  ON  LIGHT. 


brought  to  a  focus  ;  and  paper  placed  at  the 
focus  is  caused  to  smoke  and  burn.  This 
may  be  done  by  our  common  camera  with  its 
lens,  an.d  by  a  concave  mirror  of  very  mod- 
erate power. 

We  \vi!l  now  adopt  stronger  measures  with 
the  radiation  from  the  electric  lamp.  In  this 
camera  of  blackened  tin  is  placed  a  lamp,  in 
all  particulars  similar  to  those  already  em- 
ployed. But,  instead  of  gathering  up  the 
rays  from  a  carbon-point  by  a  condensing 
tens  placed  in  front  of  them,  we  gather  them 
up  by  a  concave  mirror,  silvered  in  front, 
and  placed  behind  the  carbons.  By  this 
mirror  we  can  cause  the  rays  to  issue  through 
the  orifice  in  front,  either  parallel  or  conver- 
gent. They  are  now  parallel,  and  therefore 
to  a  certain  extent  diffused.  We  place  a 
convex  lens  in  the  path  of  the  beam  ;  the 
light  is  converged  to  a  focus,  and  at  that 
focus  you  s;e  that  paper  is  not  only  pierced 
and  a  burning  ring  formed,  but  that  it  is  in- 
stantly set  ablaze.  Many  metals  may  be 
burned  up  in  the  same  way.  In  our  first  lec- 
ture the  combustibility  of  zinc  was  mentioned. 
Placing  a  strip  of  sheet-zinc  at  this  focus,  it 
is  instantly  ignited  and  burns  with  its  charac- 
teristic purple  flame.  (In  the  annexed  figure 
m  mf  represents  the  concave  mirror,  L  the 


— 4  " 


lens,  at  the  focus  C  of  which  combustion  is 
effected).  Dr.  Scoresby  succeeded  in  ex- 
ploding gunpowder  by  the  sun's  rays  con- 
verged by  large  lenses  of  ice  ;  the  same  effect 
may  be  produced  with  a  small  lens,  and  with 
a  terrestrial  source  of  heat.  In  an  iron  mould 
we  have  fashioned  this  beautiful  lens  of 
transparent  ice.  At  the  focus  of  the  lens  I 
place  a  bit  of  black  paper,  with  a  little  gun- 
cotton  folded  up  within  it.  The  paper  ignites 
and  the  cotton  explodes.  Strange,  is  it  not, 
that  the  beam  should  possess  such  heating 
power  after  having  passed  through  so  cold  a 
substance  ? 

In  this  experiment,  you  observe  that,  before 
the  beam  reaches  the  ice-lens,  it  has  passed 


'  through  a  glass  cell  containing  water.  The 
beam  is  thus  sifted  of  constituents,  which,  it 
permitted  to  fall  upon  the  lens,  would  injure 
its  surface,  and  blur  the  focus.  And  this 
leads  me  to  say  an  anticipatory  word  regard- 
ing transparency.  In  our  first  lecture  we 
entered  fully  into  the  production  of  colors 
by  absorption,  and  we  spoke  repeatedly 
of  the  quenching  of  the  rays  of  light.  Did 
this  mean  that  the  light  was  altogether 
annihilated?  By  no  means.  It  was  sim- 
ply so  lowered  in  refrangibility  as  to 
escape  the  visual  range.  //  was  converted 
into  heat.  Our  red  ribbon  in  the  green  of  the 
spectrum  quenched  the  green,  but  if  suitably 
examined  its  temperature  would  have  been 
found  raised.  Our  green  ribbon  in  the  red 
of  the  spectrum  quenched  the  red,  but  its 
temperature  at  the  same  time  was  augmented 
to  a  degree  exactly  equivalent  to  the  light  ex- 
tinguished. Our  black  ribbon,  when  passed 
through  the  spectrum,  was  found  competent 
to  quench  all  its  colors  ;  but  at  every  stage  of 
its  progress  an  amount  of  heat  was  generated 
in  the  ribbon  exactly  equivalent  to  the  light 
lost.  It  is  only  when  absorption  takes  place 
that  Jieat  is  thus  pi'oduced  ;  and  heat  is  always 
a  result  of  absorption. 

Examine  this  water,  then,  in  front  of  the 
lamp,  after  the  beam  has  passed  a  little  time 
through  it :  it  is  sensibly  warm,  and,  if  per- 
mitted to  remain  there  long  enough,  it  may 
be  made  to  boil.  This  is  due  to  the  absorp- 
tion by  the  water  of  a  portion  of  the  electric 
beam.  But  a  certain  portion  passes  through 
unabsorbed,  and  does  not  at  all  contribute  to 
the  heating  of  the  water.  Now,  ice  is  also 
transparent  to  the  latter  portion,  and  there- 
fore is  not  melted  by  it ;  hence,  by  employ- 
ing this  particular  portion  of  the  beam,  we 
are  able  to  keep  our  lens  intact,  and  to  pro- 
duce by  means  of  it  a  sharply-defined  focus. 
Placed  at  that  focus,  black  paper  instantly 
burns,  because  the  black  paper  absorbs  the 
light  which  had  passed  through  the  ice-lens 
without  absorption.  In  a  subsequent  lecture, 
we  shall  endeavor  to  penetrate  further  into 
the  physical  meaning  of  these  and  other  simi- 
lar actions.  I  may  add  to  these  illustrations 
of  heating  power,  the  ignition  cf  a  diamond 
in  oxygen,  by  the  concentrated  beam  of  the 
electric  lamp.  The  diamond,  surrounded  by 
a  hood  of  platinum  to  lessen  the  chilling  due 
to  convection,  is  exposed  at  the  focus.  It  is 
rapidly  raised  to  a  white  heat,  and  when  re- 
moved from  the  focus  continues  to  glow  like 
a  star. 

Placed  in  the  path  of  the  beam  issuing  from 
our  lamp  is  a  cell  with  glass  sides  containing 
a  solution  of  alum.  All  the  light  of  the  beam 
passes  through  this  solution.  The  beam  is 
received  on  a  powerfully  converging  mirror 
silvered  in  front,  and  is  brought  to  a  focus  by 
the  mirror.  You  can  see  the  conical  beam 
of  reflected  light  tracking  itself  through  the 
dust  of  the  room.  I  place  at  the  focus  a 
scrap  of  white  paper :  it  glows  there  with 


SIX  LECTURES  ON  LIGHT. 


37 


dazzling  btightness,  but  it  is  not  even  charred. 
On  removing  the  alum-cell,  however,  the 
paper  instantly  inflames.  There  must,  there- 
fore, be  something  in  this  beam  besides  its 
light.  The  light  is  not  absorbed  by  the 
white  paper,  and  therefore  does  not  burn  the 
paper  ;  but  there  is  something  over  and  above 
the  light  which  is  absorbed  and  which  pro- 
vokes combustion.  What  is  this  something? 
In  the  year  iSoo  Sir  William  Herschel 
passed  a  thermometer  through  the  various 
colors  of  the  solar  spectrum,  an  I  marked  the 
rise  of  temperature  corresponding  to  each 
color.  He  found  the  heating  effect  to  aug- 
ment from  the  violet  to  the  red  ;  he  did  not, 
however,  stop  at  the  red.  but  pushed  his 
thermometer  into  the  dark  space  beyond  it. 
Here  he  found  the  temperature  actually  higher 
than  in  any  part  of  the  visible  spectrum.  By 
this  important  observation,  he  proved  that 
the  sun  emitted  dark  heat-rays  which  are  en- 
tirely unfit  for  the  purposes  cf  vision.  The 
subject  was  subsequently  taken  up  by  See- 
beck,  Melloni,  Miiller,  and  others,  and  within 
the  last  few  years  it  Ins  been  found  capable 
of  unexpected  expansions  and  applications. 
A  method  has  been  devised  whereby  the  solar 
or  electric  beam  can  be  so  filtered  as  to  detach 
from  it  and  preserve  intact  this  invisible 
ultr.-red  emission,  while  the  visible  and  ultra- 
violet emissions  are  wholly  iatercepted.  We 
are  thus  enabled  to  operate  at  will  upon  the 
purely  ultra-red  waves. 

In  the  heating  of  so.id  bodies  to  incandes- 
cence this  non-visual  emission  is  the  neces- 
sary basis  of  the  visual.  A  platinum  wire  is 
stretched  in  front  of  the  table,  and  through 
it  an  electric  current  flows.  It  is  warmed  by 
the  current,  and  may  be  felt  to  be  warm  by 
the  hand  ;  it  also  emits  waves  of  heat,  but  no 
1  ght.  Augmenting  the  strength  of  the  cur- 
rent, the  wire  becomes  hotter ;  it  finally 
glows  with  a  sober  red  light-  At  this  point 
Dr.  Draper  many  years  ago  began  an  inter- 
esting investigation.  He  employed  a  vol- 
taic current  tD  heat  his  platinum,  and  he 
studied  by  means  of  a  prism  the  successive 
introduction  of  the  colors  of  the  spectrum. 
His  first  color,  as  here,  was  red  ;  then  came 
orange,  then  yellow,  then  green,  and  lastly 
all  the  shades  of  blue.  Thus  as  the  tempera- 
ture of  the  platinum  was  gradually  aug- 
mented, the  at^rns  were  caused  to  vibrate 
more  rapidly,  shorter  waves  wer3  thus  pro- 
duced, until  finally  he  obtained  the  waves 
corresponding  to  the  entire  spectrum.  As 
each  successive  color  was  introduced,  the 
colors  preceding  it  became  more  vivid.  Now, 
the  vividness,  or  intensity  of  light,  like  that 
of  sound,  depends,  not  upon  the  length  of  the 
wave,  but  on  the  amplitude  of  the  vibration. 
Hence,  as  the  red  grew  more  intense  as  the 
more  refrangible  colors  were  introduced,  we 
are  forced  to  conclude  that,  side  by  side  with 
the  introduction  of  the  shorter  waves,  we  had 
an  augmentation  of  the  amplitude  of  the 
longer  ones. 


These  remarks  apply,  not  only  to  the  vis- 
ible emission  examined  by  Dr.  Draper,  but 
to  the  invisible  emission  which  preceded 
the  appearance  of  any  light.  In  the  emis- 
sion from  the  white-hot  platinum  wire  now 
before  you  the  very  waves  exist  with  Avhich 
we  started,  only  their  intensity  has  been  in- 
creased a  thousand-fold  by  the  augmentation 
of  temperature  necessary  to  the  production 
of  this  white  light,  i  oth  effects  are  bound 
together:  in  an  incandescent  solid,  or  in  a 
molten  solid,  you  cannot  have  the  shorter 
waves  without  this  intensification  of  the 
longer  ones.  A  sun  is  possible  only  on 
these  conditions;  hence  Sir  William  Her- 
schel's  discovery  of  the  invisible  ultra-red 
solar  emission. 

The  invisible  heat,  emitted  both  by  dark 
bodies  and  by  luminous  ones,  flies  through 
space  with  the  velocity  of  light,  and  is  called 
radiant  heat.  Now,  radiant  heat  may  bj 
made  a  subtle  and  powerful  explorer  of 
molecular  condition,  and  of  late  years  it 
has  given  a  new  significance  to  the  art  of 
chemical  combination.  Take,  for  example, 
the  air  we  breathe.  It  is  a  mixture  of  oxygen 
and  nitrogen;  and  with  regard  to  radiant 
heat  it  behaves  like  a  vacuum,  being  incom- 
petent to  absorb  it  in  any  sensible  degree. 
But  permit  the  same  two  gases  to  unite 
chemically;  without  any  augmentation  of  the 
quantity  of  matter,  without  altering  the  gase- 
ous condition,  without  interfering  in  any 
way  with  the  transparency  of  the  gis,  the 
act  of  cherr.ical  uni  n  is  accompanied  by  an 
enormous  diminution  of  its  diathermancy,  or 
^erviousness  to  radiant  heat.  The  reseaiches 
which  established  this  result  also  proved  the 
elementary  gases  generally  to  be  highly 
:ransparent  to  radiant  heat.  This,  again, 
ed  to  the  proof  of  the  diathermancy  of  ele- 
mentary liquids,  like  bromine,  an  t  of  solu- 
tions of  the  elements  sulphur,  phosphorus, 
and  iodine.  A  spectrum  is  now  before  you, 
and  you  notice  that  this  transparent  bisul- 
Dhideof  carbon  has  no  effect  upon  the  colors. 
Dropping  into  the  liquid  a  few  flakes  of 
odme,  you  see  the  middle  of  the  spectrum 
cut  away.  By  augmenting  the  quantity  of 
'odine,  we  invade  the  entire  spectrum,  and 
inally  cut  it  off  altogether.  Now,  the  iodine 
,vhich  proves  itself  thus  hostile  to  the  light  is 
perfectly  transparent  to  the  ultra-red  emis- 
sion with  which  we  have  now  to  deal.  It, 
herefore,  is  to  be  our  ray-filter. 

Placing  the  alum-cell  again  in  front  of  the 
electric  lamp,  we  assure  ourselves,  as  before, 
of  the  utter  inability  of  the  concentrated 
Jight  to  fire  white  paper.  By  introducing  a 
cell  containing  the  solution  of  iodine,  the 
ight  is  entirely  cut  off.  On  remov- 
ng  the  alum-cell,  the  paper  at  the  dark  focus 
s  im  tantly  set  en  fire.  Black  paper  is  more 
bsorbent  than  white  for  these  ultra-red  rays; 
and  the  consequence  is,  that  with  it  the  sud- 
denness and  vigor  of  the  combustion  are  aug- 
mented. Zinc  is  burnt  up  at  the  same  place, 


38 


SIX  LECTURES  ON  LIGHT 


while  magnesium  ribbon  bursts  into  vivid 
combustion  A  sheet  of  platinized  platinum 
placed  at  the  focus  is  heated  to  whiteness. 
Looked  at  through  a  prism,  the  white-hot 
platinum  yields  all  the  colors  of  the  spectrum. 
Before  impinging  upon  the  platinum,  the 
waves  were  of  too  slow  recurrence  to  awaken 
vision;  by  the  atoms  of  the  platinum,  these 
long  and  sluggish  waves  are  in  part  broken 
up  into  shorter  ones,  being  thus  brought 
within  the  visual  range.  At  the  other  end  of 
the  spectrum,  Stokes,  by  the  interposition  of 
suitable  substances,  lowered  the  refrangibil- 
ity  so  as  to  render  the  non- visual  rays  visual, 
and  to  this  change  he  gave  the  name  of 
Fluorescence..  Here,  by  the  intervention  of 
the  platinum,  the  refrangibility  is  raised,  so 
as  to  render  the  non-visual  visual,  and  to 
this  change  we  give  the  name  of  Calorcs- 
cence. 

At  the  perfectly  invisible  focus  where  these 
effects  are  produced,  the  air  may  be  as  cold 
as  ice.  Air,  as  already  stated,  does  not  ab- 
sorb the  radiant  heat,  and  is  therefore  not 
warmed  by  it.  Place  at  the  focus  the  most 
sensitive  air-thermometer  :  it  is  not  affected 
by  the  heat.  Nothing  could  more  forcibly 
illustrate  the  isolation,  if  I  may  use  the  term, 
of  the  luminiferous  ether  from  the  air.  The 
\vave-motion  of  the  one  is  heaped  up,  without 
sensible  effect,  upon  the  other.  I  may  add 
that,  with  suitable  precautions,  the  eye  may 
be  placed  in  a  focus  competent  to  heat  plati- 
num to  vivid  redness,  without  experiencing 
any  damage,  or  the  slightest  sensation  either 
of  light  or  heat. 

These  ultra-red  rays  play  a  most  important 
part  in  Nature.  I  remove  the  iodine  filter, 
and  concentrate  the  total  beam.  A  test-tube 
containing  water  is  placed  at  the  focus  :  it 
immediately  begins  to  sputter,  and  in  a  min- 
ute or  two  it  boils.  What  boils  it?  Placing 
the  alum  solution  in  front  of  the  lamp,  the 
boiling  instantly  ceases.  Now,  the  alum  is 
pervious  to  all  the  luminous  rays;  hence  it 
cannot  be  these  rays  that  caused  the  boiling. 
I  now  introduce  the  iodine,  and  remove  the 
alum  ;  vigorous  ebullition  immediately  re- 
commences. So  that  we  here  fix  upon  the 
invisible  ultra-red  rays  the  heating  of  the  wa- 
ter. We  are  enabled  now  to  understand  the 
momentous  part  played  by  these  rays  in  Na- 
ture. It  is  to  them  that  we  owe  the  warming 
and  the  consequent  evaporation  of  the  tropi- 
cal ocean  ;  it  is  to  them,  therefore,  that  we 
owe  our  rains  and  snows.  They  are 
absorbed  close  to  the  surface  of  the 
ooean,  and  warm  the  superficial  water 
while  the  luminous  rays  plunge  to 
great  depths  without  producing  any 
sensible  effect.  Further,  here  is  a  large  flask 
containing  a  freezing  mixture.  The  aqueous 
vapor  of  the  air  has  been  condensed  and 
frozen  on  the  flask,  which  is  now  covered 
with  a  white  fur.  Introducing  the  alum-cell, 
we  place  the  coating  of  hoar-frost  at  the  in- 
tensely luminous  focus  ;  not  a  spicula  of  the 


frost  is  melted.  Introducing  the  iodine-cell, 
j  and  removing  the  alum,  a  broad  space  of  the 
frozen  coating  is  instantly  removed.  Hence 
we  infer  that  the  ice  which  feeds  the  Rhone, 
the  Rhine,  and  cthei  rivers  which  have 
glaciers  for  their  sources,  is  released  from  its 
imprisonment  upon  the  mountains  by  the 
invisible  ultra- red  rays  of  the  sun. 

The  growth  of  science  is  organic.  The 
end  of  to-day  becomes  to-morrow  the  means 
to  a  remoter  end.  Every  new  discovery  is 
immediately  made  the  basis  of  other  discov- 
eries, or  of  new  methods  of  investigation. 
About  fifty  years  ago,  CErsted,  of  Copen- 
hagen, discovered  the  deflection  of  a  mag- 
netic needle  by  an  electric  current ;  and 
Thomas  Seebeck,  of  Berlin,  discovered  that 
electric  currents  might  be  derived  from  heat. 
Soon  afterwards  these  discoveries  were  turned 
to  account  by  Nobili  and  Melloni  in  the  con- 
struction of  an  apparatus  which  has  vastly 
augmented  our  knowledge  of  radiant  heat. 
The  instrument  is  here.  It  is  called  a  thermo- 
electric pile;  and  it  consists  of  thin  bars  of 
bismuth  and  antimony  soldered  together  in 
pairs  at  their  ends,  but  separated  from  each 
other  elswhere.  From  the  ends  of  this  '  'pile" 
wires  pass  to  a  coil  of  covered  wire,  within 
and  above  which  are  suspended  two  magnetic 
needles  joined  to  a  rigid  system,  and  carefully 
defended  from  currents  of  air.  The  heat, 
then,  acting  on  the  pile,  produces  an  electric 
current  ;  t'he  current,  passing  through  the 
coil,  deflects  the  needles,  and  the  magnitude 
of  the  deflection  may  be  made  a  measure  of 
the  heat.  The  upper  needle  moves  over  a 
graduated  dial  far  too  small  to  be  seen.  It 
is  now,  however,  strongly  illuminated.  Above 
it  is  a  lens  which,  if  permitted,  would  form 
an  image  of  the  needle  and  dial  upon  the 
ceiling,  where,  however,  it  could  not  be  con- 
veniently seen.  The  beam  is  therefore  re- 
ceived upon  a  looking-glass,  placed  at  the 
proper  angle,  which  throws  the  image  upon 
the  screen.  In  this  way  the  motions  of  this 
small  needle  may  be  made  visible  to  you  all. 

The  delicacy  of  this  instrument  is  such 
that  in  a  room  like  this  it  is  exceedingly  dif- 
ficult to  work  with  it.  My  assistant  stands 
several  ieet  off.  I  turn  the  pile  towards  him: 
the  heat  from  his  face,  even  at  this  distance, 
produces  a  deflection  of  90°.  I  turn  the  in- 
strument towards  a  distant  wall,  which  I 
udge  to  be  a  little  below  the  average  temper- 
ature of  the  room.  The  needle  descends  and 
passes  to  the  other  side  of  zero,  declaring  by 
Lhis  negative  deflection  that  the  pile  feels  the 
chill  of  the  wall .  Possessed  of  this  instrument, 
of  our  ray-filter,  and  of  our  large  Nicol  prisms, 
we  are  in  a  condition  to  investigate  a  subject 
of  great  philosophical  interest,  and  which 
'ong  engaged  the  attention  of  some  of  our 
foremost  scientific  workers,  Forbes  being  the 
first  successful  one — the  substantial  identity 
of  light  and  radiant  heat. 

That  they  are  identical  in  all  respects  can- 
not of  course  be  the  case,  for  if  they  vr£  re 


SIX  LECTURES  ON  LIGHT. 


they  would  act  in  the  same  manner  upon  all 
instruments,  the  eye  included.  '1  he  identity 
meant  is  such  as  subsists  between  one  color 
and  another,  causing  them  to  behave  alike  ^s 
regards  reflection,  refraction,  double  refrac- 
tion, and  polarization.  As  regards  reflection, 
\ve  may  employ  the  looking-glass  used  in  our 
first  lecture.  ^larking  any  point  in  the  track 
of  the  reflected  beam,  and  cutting  off  the  | 
light  by  the  iodine,  on  placing  the  pile  at  the 
marked  point,  the  needle  immediately  starts 
aside.  This  is  tru?  for  every  position  of  the 
mirror.  So  that  both  for  light  and  heat  the 
same  lav/  of  reflection  holds  good;  for  both 
of  them  also  the  angular  velocity  of  the  re- 
flected beam  is  twice  that  of  the  reflecting 
mirror.  Receiving  the  beam  on  a  concave 
mirror,  it  is  gathered  up  into  a  cone  of  re- 
flected :ight;  marking  the  apex  of  the  cone, 
and  cutting  off  the  light,  a  moment's  expo- 
sure of  the  pile  at  the  marked  point  produces 
a  violent  deflection  of  the  needle.  (See  Fig. 
23,  where  m  111  is  the  mirror,  P  the  pile,  and 
T  the  opaque  solution.) 

This  beam  of  light  now  enters  a  right- 
angled  prism  and  is  reflected  at  the  hvpothe- 
nuse,  i:i  a  direction  perpendicular  to  its  for- 
mer one.  The  reflection  here  is  total.  Cut- 
ling  off  the  light,  we  prove  the  reflection  of 
the  heat  to  be  total  also.  The  formation  of 


invisible  images  by  lenses  and  mirrors  may 
also  be  demonstrated.  Concentrating  the 
beam,  and  cutting  off  the  light,  at  the  dark 
focus  the  carbon-points  burn  their  images 
through  a  sheet  of  black  paper.  Placing  a 
sheet  of  platinized  platinum  at  the  focus, 
when  the  concentration  is  strong  an  incan- 
descent image  of  the  points  is  immediately 
stamped  upon  the  platinum. 

And  now  for  polarization  and  its  attendant 
phenomena.     Crossing  our  two  Nicol  prisms, 


B,  C,  Fig.  24,  and  placing  ou  pile  D  be- 
hind the  analyzer,  neither  heat  nor  light 
reaches  it;  the  needle  remains  undeflect- 
cd.  Introducing  the  iodine,  the  slight- 
est turning  of  either  prism  causes  the 
heat  to  pass,  and  to  announce  it- 
self by  the  deflection  of  the  needle.  Like 
light,  therefore,  heat  is  polarized.  Crossing 
the  Nicols  ag^in,  the  heat  is  intercepted  and 


the  needle  returns  to  zero.  Plunging  into 
the  dark  space  between  the  prisms  our  plate 
of  mica,  the  needle  instantly  starts  off,  show- 
ing that  the  mica  acts  upon  the  heat  as  it  did; 
upon  the  light  :  we  have  in  both  cases  the 
same  resolution  and  recompounding  of  vi- 
brations. Removing  the  mica,  the  needle 
falls  to  zero  ;  but,  on  introducing  a  plate  of 
quartz  between  the  prisms,  the  consequent; 
deflection  declares  the  circular  polarization  of 
the  heat.  For  double  refraction  it  is  neces- 
sary that  our  images  should  not  be  too  large- 
and  diluted  :  here  are  the  two  disks  pro- 
duced by  the  splitting  of  the  beam  in  Ice- 
land spar.  Marking  the  positions  of  the  disks, 
and  cutting  off  the  light,  the  pile  finds  in  its 
places  two  heat-images.  The  needle  now- 
stands  near  90°,  and,  on  turning  the  spar,, 
the  deflection  remains  constant.  Transfer- 
ring the  pile  to  the  other  image,  the  cU-fiec-. 
lion  of  90°  is  maintained  ;  but  on  turning 
the  spar  the  needle  now  falls  to  zero.  The 
reason  is  manifest.  Permitting  the  light  to- 
pass,  we  find  the  luminous  disk  at  some  dis- 
tance from  the  pile.  We  are  dealing,  in 
fact,  with  the  extraordinary  *bcam  which 
rotates  round  the  ordinary  So  that  fr.~  >  t-at 
as  well  as  for  light  we  have  double  refraction, 
and  also  an  ordinary  and  extraordinary  ray. 
(In  the  adjacent  figure,  which  shows  the  ex- 
perimental arrangement,  N  is  the  nozzle  cf 


40 


SIX  LECTURES  ON  LIGHT. 


the  electric  lamp,  L  a  converging  lens,  B  the 
bircfracting  spar,  and  P  the  thermo-electric 
pile.) 

If  time  permitted  we  might  finish  the  series 
of  demonstrations  by  magnetizing  a  ray  of 
heat  as  we  magnetized  a  ray  of  light. 

We  have  finally  to  determine  the  position 
and  magnitude  of  the  invisible  radiation 
which  produces  these  results.  For  this  pur- 
pose we  employ  a  particular  form  of  the 
thermo-electric  pile.  Its  face  is  a  rectangle, 
which  by  movable  side-pieces  can  be  ren- 
dered as  narrow  as  desirable.  Throwing  a 
concentrated  spectrum  upon  a  screen, 
by  means  of  an  endless  screw,  we  move  this, 
rectangular  pile  through  the  entire  spectrum. 
Its  surface  is  blackened  so  that  it  absorbs  all 
the  light  incident  upon  it,  converting  it  into 


a  curve  which  exhibits  the  distribution  of 
heat  in  our  spectrum.  It  is  represented  in 
the  adjacent  figure.  Beginning  at  the  blue, 
the  curve  rises,  at  first  very  gradually;  then, 
as  it  approaches  the  red  more  rapidly,  the 
line  CD  representing  the  strength  of  the  ex- 
treme red  radiation.  Beyond  the  red  it  shoots 
upwards  in  a  steep  and  massive  peak  to  B, 
whence  it  falls,  rapidly  for  a  time,  and  after- 
wards gradual'y  fading  from  the  perception 
of  the  pile.  This  figure  is  the  result  of 
more  than  twelve  careful  series  of  measure- 
ments, for  each  of  which  the  curve  was  con- 
structed. On  superposing  all  these  curves, 
a  satisfactory  agreement  was  found  to  exist 
between  them.  So  that  it  may  safely  be 
concluded  that  the  areas  of  the  dark  and 
white  spaces  respectively  represent  the  rela- 


FIG.  25. 


heat,  and  thus  enabling  it  to  declare  its  power 
by  the  deflection  of  the  magnetic  needle. 

When  this  instrument  is  brought  to  the 
violet  end  of  the  spectrum,  the  heat  is  found 
to  be  almost  insensible.  As  the  pile  grad- 
ually moves  from  the  violet  towards  the  red, 
it  encounters  a  gradually  augmenting  heat. 
The  red  itself  possesses  the  highest  heating 
power  of  all  the  colors  of  the  spectrum. 
Pushing  the  pile  into  the  dark  space  beyond  the 
red,  the  heat  rises  suddenly  in  intensity, and.at 
some  distance  beyond  the  red,  attains  a  max- 
imum. From  this  point  the  heat  falls  some- 
what more  rapidly  than  it  rose,  and  afterwards 
gradually  fades  away.  Drawing-  an  hori- 
zontal line  to  represent  the  length  of  the 
spectrum,  and  erecting  along  it,  at  various 
points,  perpendiculars  proportional  in  length 
to  the  heat  existing  at  those  points,  we  obtain 


I  tive  energies   of    the    visible    and  invisible 
I  radiation      The  one  is  7.7  times  the  other. 

But  in  verification,  as  already  stated,  con- 
sists the  strength  of  science.     Determining 
in  the  first  place  the  total  emission  from  the 
electric  lamp ;  then  by  means  of  the  iodine 
filter  determining  the  ultra-red  emission  ;   the 
difference  between  both  gives  the  luminous 
emission.     In  this  way,  it  was  found  that  the 
energy  of  the  invisible  emission  is  eight  times 
that  of  the  visible.     No  two  methods  could 
be  more  opposed  to  each  other,  and  hardly 
j  any  two  results  could  better  harmonize.     I 
I  think,  therefore,  you  may  rely  upon  the  ac- 
j  curacy  of  the  distribution  of  heat  here  as- 
signed to  the  prismatic  spectrum  of  the  elec- 
i  trie  light.     There   is  nothing  vague  in  the 
j  mode  of  investigation,   nor  doubtful  in  its 
conclusions. 


SIX  LECTURES  ON  LIGHT. 


LECTURE   VI. 

of   Spectrum  Analysis:    Solar  Chemistry: 
Summary  and  Conclusions. 

We  have  employed,  as  our  source  of  light 
in  these  lectures,  the  ends  of  two  rods  of  coke 
rendered  incandescent  by  electricity.  Coke 
is  particularly  suitable  for  this  purpose,  be- 
cause it  can  bear  intense  heat  without  fus  on 
or  vaporization.  It  is  also  black,  which 
helps  the  light;  for,  other  circumstances  be- 
ing equal,  as  Ihown  experimentally  by  Bal- 


four  Stewart,    the    blacker    the    body    the 
brighter  will  be  its  light  when  incandescent. 
Still,  refractory  as  carbon  is,  if  we  closely  ex-' 
amine  our  voltaic  arc,  or  stream  of  light  be- 
tween the  carbon-points,  we  should  find  there 
incandescent   carbon-vapor.     We  might  also 
|  detach  the  light  of  this  vapor  from  the  more 
!  dazzling  light  of  the  solid  points,  and  obtain 
!  its  spectrum.     This  would   be  not  only  les.i 
'  brilliant,  but  of   a  totally  different  character 
j  from  the  spectra   that  we  have  already  seen. 
i  Instead  of  being  an    unbroken  succession  t>f 
'  colors  from  red  to  violet,    the   carbon-vapor 


42 


SIX  LECTURES  ON  LIGHT, 


would  yield  a  few  bands  of  color  with  spaces 
of  darkness  between  them. 

What  is  true  of  the  carbon  is  true  in  a  still 
more  striking  degree  of  the  metals,  the  most 
refractory  of  which  can  be  fused,  boiled,  ind 
reduced  to  vapor  by  the  electric  current. 
From  the  incandescent  vapor  the  light,  as  a 
general  rule,  flashes  in  groups  of  rays  of 
definite  degrees  of  refrangibility,  spaces  ex- 
isting between  group  and  group,  which  are 
unfilled  by  rays  of  any  kind.  But  the  con- 
templation of  the  facts  will  render  this  sub- 
ject more  intelligible  than  words  can  make  it. 
Within  the  camera  is  now  placed  a  cylinder 
of  carbon  hollowed  out  at  the  t<"p  to  receive 
a  bit  of  metal;  in  the  hollow  is  placed  a  frag- 
ment of  the  m;tal  thallium,  and  now  you  see 
the  arc  of  incandescent  thallium-vapor  upon 
the  screen.  It  is  of  a  beautiful  green  color. 
What  is  the  meaning  of  that  green?  We 
answer  the  question  by  subjecting  the  light 
to  prismatic  analysis.  Here  you  have  its 
spectrum,  consisting  of  a  single  refracted 
band.  Light  of  one  degree  of  icfrangibility, 
and  that  corresponding  to  green,  is  emitted 
by  the  thallium-vapor. 

We  will  now  remove  the  thallium  and  put 
a  bit  of  silver  in  its  place.  The  arc  of  silver 
is  not  to  be  distinguished  from  that  of  thal- 
lium; it  is  not  only  green,  like  the  thallium- 
vapor,  but  the  same  shade  cf  green.  Are 
they,  then,  alike?  Prismatic  analysis  en- 
ables us  to  answer  the  question.  It  is  per- 
fectly impossible  to  confound  the  spectrum 
<  f  incandescent  silver  vapor  with  that  of 
thallium.  Here  are  two  green  bands  instead 
of  one.  Adding  to  the  silver  in  our  camera 
a  bit  of  thallium,  we  obtain  the  light  of  both 
metals,  and  you  see  that  the  green  of  the  thal- 
lium lies  midway  between  the  two  greens  of 
the  silver.  Hence  this  similarity  of  color. 

But  you  observe  another  interesting  fact. 
The  thallium  band  is  now  far  brighter  than 
the  silver  bands  ;  indeed,  the  latter  have  won- 
derfully degenerated  since  the  bit  of  thallium 
was  put  in.  They  are  not  at  all  so  bright  as 
they  were  at  first,  and  for  a  reason  worth 
knowing.  It  is  the  resistance  offered  to  the 
passage  of  the  electric  current  from  carbon 
to  carbon  that  calls  forth  the  power  of  the 
current  to  produce  heat.  If  the  resistance 
were  materially  lessened,  the  heat  would  be 
materially  lessened ;  and,  if  all  resistance 
were  abolished,  there  would  be  no  heat  at 
all.  Now,  thallium  is  a  much  more  fusible 
and  vaporizable  metal  than  silver;  and  its 
vapor  facilitates  the  passage  of  the  current  to 
such  a  degree  as  to  render  it  almost  incom- 
petent to  vaporize  the  silver.  But  the  thal- 
lium is  gradually  consumed  ;  its  vapor  di- 
ininishes,  the  resistance  rises,  until  finally 
you  see  the  two  silver  bands  as  brilliant  as 
they  were  at  first.  The  three  bands  cf  the 
two  metals  are  now  of  the  same  sensible 
brightness. 

We  have  in  these  bands  a  perfectly  unal- 
terable characteristic  of  these  two  metals. 


You  never  get  other  bands  than  these  two 
green  ones  from  the  silver,  never  other  than 
the  single  green  band  from  the  thallium, 
never  olher  than  the  three  green  bands  from 
the  mixture  of  both  metals.  Every  known 
metal  has  its  bands,  and  in  no  known  case 
are  the  bands  of  two  different  metals  al;ke. 
Hence  these  spectra  may  be  made  a  test  for 
the  presence  or  absence  of  any  particular 
metal.  If  we  pass  from  the  metals  to  their 
alloys,  we  find  no  confusion.  Copper  gives 
us  green  bands,  zinc  gives  us  blue  and  red 
bands  ;  brass,  an  alloy  of  copper  and  zinc, 
gives  us  the  bands  of  both  metals,  perfectly 
unaltered  in  position  or  character. 

But  we  are  not  confined  to  the  metals  ;  the 
salts  of  these  metals  yield  the  bands  of  the 
metals.  Chemical  union  is  ruptured  by  a 
sufficiently  high  heat,  the  vapor  of  the  metal 
is  set  free  and  yields  its  characteristic  bands. 
The  chlorides  of  the  metals  arc  particularly 
suitable  for  experiments  of  this  character. 
Common  salt,  for  example,  is  a  compound  of 
chlor.nc  and  sodium  ;  in  the  electric  lamp,  it 
yields  the  spectrum  of  the  metal  sodium. 
The  chlorides  of  lithium  and  cf  strontium 
yield  in  like  manner  the  bands  of  these 
metals.  When,  therefore,  Bunsen  and  Kirch- 
hoff,  the  celebrated  founders  of  spectrum 
analysis,  after  having  established  by  an  ex- 
haustive examination  the  spectra  of  all  known 
substances,  discovered  a  spectrum  containing 
bands  different  from  any  known  bands,  they 
immediately  inferred  the  existence  of  a  new 
metal.  They  were  operating  at  the  time 
upon  a  residue  obtained  by  evaporating  one 
of  the  mineral  waters  of  Germany.  In  that 
water  they  knew  the  new  metal  was  con- 
cealed, but  vast  quantities  of  it  had  to  be 
evaporated  before  a  residue  could  be  obtained 
sufficiently  large  to  enable  ordinary  chemistry 
to  grapple  with  the  metal.  But  they  hunted 
it  down,  and  it  now  stands  among  chemical 
substances  as  the  metal  Rubidium*  They 
subsequently  discovered  a  second  metal, 
whic:i  they  called  Casium.  Thus,  having 
first  placed  spectrum  analysis  on  a  safe  foun- 
dation, they  demonstrated  its  capacity  as  an 
agent  of  discovery.  Soon  afterwards  Mr. 
Crookes,  pursuing  this  same  method,  ob- 
tained the  salts  of  the  thallium  which  yielded 
that  bright  monoc-  romatic  green  band.  The 
metal  itself  was  first  isolated  by  a  French 
chemist. 

All  this  relates  to  chemical  discovery  upon 
earth,  where  the  materials  are  in  our  own 
hands.  But  Kirchhoff  showed  how  spectrum 
analysis  might  be  applied  to  the  investigation 
cf  the  sun  and  stars,  and  on  1  is  way  to  this 
result  he  solved  a  problem  which  had  been 
long  an  enigma  to  natural  philosophers.  A 
spectrum  is  pun  in  which  the  colors  do  not 
overlap  each  other.  We  purify  the  spectrum 
by  making  our  slit  narrow  and  by  augment- 
ing the  number  of  our  prisms.  When  a  pure 
spectrum  of  the  sun  has  been  obtained  in 
this  way  it  is  found  to  be  furrowed  by  in- 


SIX  LECTURES  ON  LIGHT. 


43 


O 


numerable  dark  lines.  Four  of  them  were 
lirst  seen  by  Dr.  \Vollaston,  but  they  were 
afterwards  multiplied  and  me.*sured  by  Fraun-. 
hofer  with  such  masterly  skill  that  they  are 
now  universally  known  as  Fraunhofer's  lines. 
To  give  an  explanation  of 
these  lines  was.  as  I  have  said, 
a  problem  which  long  chal- 
lenged the  attention  of  philos- 
ophers. (The  principal  lines 
are  lettered  according  to  Fraun- 
hofer  in  the  annexed  sketch  of 
the  solar  spectrum.  A,  it  may 
be  stated  stands  near  the  ex- 
treme red,  and  J  near  the 
extreme'violet.) 

Now,  Kirchhoff  had  made 
thoroughly  clear  to  his  mind 
the  priric  pies  which  linked  to- 
gether the  emission  of  light 
and  the  absorption  of  light  ; 
he  had  proved  their  insepara- 
bility  for  each  particular  kind 
of  light  and  heat.  He  had 
proved,  for  every  specific  ray 
of  the  spectrum,  the  doctrine 
that  the  body  emitting  any  ray 
absorbed  with  special  energy  a 
ray  of  the  same  refrangibility. 
Consider,  then,  the  elfect  of 
knouhdge,  such  as  you  now 
possess,  upon  a  mind  prepared 
like  that  of  Kirchhoff.  We 
have  seen  the  incandescent  va- 
pors of  metals  emitting  defi- 
nite groups  of  rays  ;  accord- 
ing to  Kirchhoff 's  principle, 
those  vapors,  if  crossed  by  solar 
light,  ought  to  absorb  :  ays  of 
the  same  refrangibility  as  those 
which  they  emit.  He  proved 
this  to  be  the  case ;  he  was 
able,  by  the  interposition  of  a 
vapor,  to  cut  out  of  the  solar 
spectrum  the  band  correspond- 
ing in  color  to  that  vapor. 
Now.  the  sun  possesses  a  pho- 
tosphere, or  vaporous  enve- 
lope— doubtless  mixed  with  vi- 
olently agitated  clouds — and 
Kirchhoff  saw  that  the  power- 
ful rays  coming  from  the  solid,  or  the 
molten  nucleus  of  the  sun,  must  be  inter- 
cepted by  this  vapor.  One  dark  band  of  j 
Fraunhof  r,  for  example,  occurs  in  the 
yellow  of  the  spectrum.  Sodium  vapor  h 
demonstrably  competent  to  produce  that  dark 
band  ;  hence  Kirchhoff  inferred  the  exist- 
ence of  sodium-vapor  in  the  atmosphere  of 
,the  sun.  In  the  case  of  metals,  which  emit 
a  large  number  of  bands,  the  absolute  coi  - 
c  dence  of  every  bright  band  of  the  metal 
with  a  dark  Fraunhofer  line,  raises  to  the 
highest  degree  of  certainty  the  inference  that 
the  metal  is  present  in  the  atmosphere  of  the 
sun.  In  this  way  solar-  chemistry  was  found- 
ei  on  spectrum  analysis. 


FIG.  27. 


But  let  me  not  skim,  so  lightly  over  this 
great  subject.  I  have  spoken  of  emis- 
sion and  absorption,  and  of  the  link  that 
binds  them.  Let  me  endeavor  to  make  plain 
to  you,  through  the  analogy  of  sound,  their 
physical  meaning.  I  draw  a  fiddle-bow  across 
this  tuning-fork,  and  it  immediately  fills  the 
room  with  a  musical  sound;  this  may  be  re- 
garded as  the  radiation  or  emission  of  sound 
from  the  fork.  A  few  days  ago,  on  sounding 
this  fork,  I  noticed  that,  when  its  vibrations 
were  quenched,  the  sound  seemed  to  be  con- 
tinued, though  more  feebly;  The  sound  ap- 
peared to  come  from  under  a  distant  table, 
where  stood  a  number  of  tuning-forks  of  dif- 
ferent sizes  and  rates  of  vibration.  One  of 
these,  and  one  only,  had  been  started  by  the 
fork,  and  it  was  one  whose  rate  of  vibration 
was  the  same  as  that  of  the  fork  which 
started  it.  This  is  an  instance  of  the  ab- 
sorption of  sound  of  one  fork  by  -another. 
Placing  two. forks  near  each  other,  sweeping 
the  bow  over  one  of  them,  and  then  quench- 
ing the  agitated  fork,  the  other  continues  to 
sound.  Placing  a  cent-piece  on  each  prong 
of  one  of  the  forks,  we  destroy  its  perfect 
synchronism  \\ith  the  other,  and  then  no  com- 
munication of  sound  from  the  one  to  the  other 
is  possible. 

1  will  now  do  with  //^vfc/what  has  been  here 
done  with  sound.  Placing  a  tin  spoon  con- 
taining sodium  in  a  Bunsen's  flame,  we  ob- 
tain this  intensely  yellow  light,  which  corn-- 
sponds  in  refrangibility  with  the  yellow  band 
of  the  spectrum.  Like  our  tuning-fork,  it 
emits  waves  at  a  special  period.  I  will  send 
the  white  light  from  our  lamp  through  that 
flame,  and  prove  before  you  that  the  yellow 
flame  intercepts  the  yellow  of  the  spectrum 
S  S,  Fig.  28;  in  other  words,  absorbs  wrves 
of  the  same  period  as  its  own,  thus  produc- 
ing, to  all  intents  and  purposes,  a  da:k 
Fraunhofer's  band  in  the  place  of  the  yellow. 
(A  Bunsen's  flame  contained  within  the  chim- 
ney C  is  placed  in  front  of  the  lamp  L.  Tha 
tin  spoon  with  its  pellet  of  sodium  is  plunged 
into  the  flame.  Vivid  combustion  soon  seU 
in,  and,  when  it  does,  the  yellow  of  the  speo 
trum,  at  D,  is  furrowed  by  a  dark  band. 
Withdrawing  and  introducing  the  sodium. 
flame  in  rapid  succession,  the  sudden  disap- 
pearance a  id  reappearance  of  the  strip  of 
darkness  arc  observed). 

Mentally,  as  well  as  physically,  every  age 
of  the  world  is  the  outgrowth  and  offsprin^ 
of  all  preceding  ages.  Science  proves  itself 
to  be  a  genuine  product  of  Nature  by  grow- 
ing according  to  this  law.  We  have  no  so- 
lution of  continuity  here.  Every  great  dis- 
covery has  been  duly  prepared  for  in  two 
ways:  first,  by  other  discoveries  which  form 
its  prelude;  and,  secondly,  through  the 
sharpening,  by  exercise,  of  the  intellectual 
instrument  itself.  Thus  Ptolemy  grew  ouij 
of  Hipparchus,  Copernicus  out  of  both,  Kep- 
ler out  of  all  three,  and  Newton  out  of  all 
the  four.  Newton  did  not  rise  suddenly  Jrora 


44. 


SIX  LECTURES  ON  LIGHT. 


the  sea-level  bf  the  intellect  to  his  amazing 
elevation.  At  the  time  that  he  appeared,  the 
table-land  of  knowledge  was  already  high. 
He  juts,  it  is  true,  above  the  table-land,  as  a 
massive  peak;  still  he  is  supported  by  it,  and 
a  great  part  of  his  absolute  height  was  the 
height  of  humanity  in  his  time.  It  is  thus 


»'rn  thediscowies  of  Kirchhoff.  Much  had 
beta  previously  accomplished;  this  he  mas- 
tered, and  the*:  by  the  force  of  individual 
genius  went  beyond  it.  He  replaced  uncer- 
tainty by  certaiuty,  vagueness  by  definite- 
ness,  confusion  Ly  oxler;  and  I  do  not  think 
that  Newton  has  a  s:i»  er  claim  to  the  discov- 
ciies  that  have  made  h';s  name  immortal  than 
Kirchhoff  has  to  the  credit  of  gathering  up  the 
fragmentary  knowledge  of  his  time,  of  vastly 
extendir£  it,  and  of  infusing  into  it  the  life 
of  great  principles.  Splendid  results  have 
since  been  obtained  with  whk  ii  the  names  of 
Janssen,  Muggins,  I,ocl:>?r,  Respighi, 
Young,  and  others,  are  honorably  associated, 
but,  splendid  as  they  are,  they  are  but  the 
sequel  and  application  of  the  principles  es- 
tablished in  his  Heidelberg  laboratory  by  the 
celebrated  German  investigator. 


SUMMARY  AND  CONCLUSION 

My  desire  in  these  lectures  hz?s  bta.n  to 
show  you,  with  as  little  breacn  of  continuity 
as  possible,  the  past  growth  and  p/esent  as- 
pect of  a  department  cf  science,  ;'n  which 
have  labored  some  of  th^  gre;  test  i  itellccts 
the  world  has  ever  seen.  My  f  "lend  Profes- 
sor Henry,  in  introducing  me  at  Washington, 
spoke  of  me  as  an  apostle;  but  theonh  apos- 
tolate  that  I  intended  to  fulfil  was  to  place, 


in  plain  words,  my  subject  before  you,  and  to 
permit  its  own  intrinsic  attrsctions  to  act  up- 
on your  minds.  In  the  \vayof  experiment,  I 
have  tried  to  give  you  the  best  which,  under 
the  circumstances,  could  be  provided;  but  I 
have  sought  to  confer  on  each  experiment  a 
distinct  intellectual  vaLie,  for  experiments 
ought  to  be  the  representatives  and  exposi- 
tors of  thought — a  language  addressed  to  the 
eye  as  spoken  words  are  to  the  ear.  In  as- 
sociation with  its  context,  nothing  is  more 
impressive  or  instructive  than  a  fit  experi- 
ment; but,  apart  from  its  context,  it  rather 
suits  the  conjuror's  purpose  of  surprise  than 
that  purpose  of  education  which  ought  to  be 
the  ruling  motive  of  the  scientific  man. 

And  now  a  brief  summary  of  our  work 
will  not  be  out  of  place.  Our  present  mas- 
tery over  the  laws  and  phenomena  of  light 
has  its  origin  in  the  desire  of  man  to  know. 
We  have  seen  the  ancients  busy  whh  this 
problem,  but,  like  a  child  who  uses  his  aims 
aimlessly  for  want  of  the  necessarv  muscular 
exercise,  so  these  early  men  speculated  vagut- 
ly  and  confusedly  regarding  light,  not  having 
as  yet  the  discipline  needed  to  give  clearness 
to  their  insight,  and  firmness  to  their  grasp 
of  principles.  They  assured  themselves  of 
th~  rectilineal  propogation  of  light,  and  that 
the  angle  of  incidence  was  equal  to  the  angle 
of  reflection.  For  more  than  a  thousand 
years — I  might  say,  indeed,  for  more  than 
ritteen  hundred  years  subsequently  —  the 
scientific  intellect  appears  as  if  smitten  with 
paralysis,  the  fact  being  that,  during  this 
time,  the  mental  force,  which  might  have  run 
in  the  direction  of  science,  was  diverted  in- 
;o  other  directions. 

The  course  of  investigation  as  regards 
light  was  resumed  in  1100  by  an  Arabian 
philosopher  named  Alhazan.  Then  it  was 
taken  up  in  succession  by  Roger  Bacon,  Vi- 
tellio,  and  Kepler.  These  men,  though  fail- 
ing to  detect  the  principle  which  ru.ed  the 
facts,  kept  the  fire  of  investigation  constantly 
burning.  Then  came  the  fundamental  dis- 
covery of  Snell,  that  corner-stone  of  optics, 
as  I  have  already  called  it,  and  immediately 
afterward  we  have  the  application  by  Des- 
cartes of  Snell's  discovery  to  the  expianaticn 
of  the  rainbow.  Then  came  Newton's 
crowning  experiments  on  the  analysis  and 
i  synthesis  of  white  light,  by  which  it  was 
proved  to  be  compounded  of  various  kinds 
of  light  of  different  degrees  of  refrangibiiity. 

In  1676  an  impulse  was  given  to  optics  by 
astronomy.  In  that  year  Olaf  Roemer,  a 
learned  Dane,  was  engaged  at  the  Observa- 
tory of  Paris  in  observing  the  eclipses  of  Ju- 
piter's moons.  He  converted  them  into  so 
many  signal-lamps,  quenched  when  they 
plunged  into  the  shadow  of  the  planet,  and 
relighted  when  they  emerged  from  the  shadow. 
They  enabled  him  to  prove  that  light 
requires  time  to  pass  through  space,  and  to 
assign  to  it  the  astounding  velocity  of  190,- 
ooo  miles  a  second.  Then  came  the  English 


SIX  LECTURES  ON  LIGHT. 


45 


astronomer, '  Bradley,  who  noticed  that  the 
fixed  stars  did  not  really  appear  to  be  fixed, 
but  describe  in  the  heavens  every  year  a  little 
orbit  resembling  the  earth's  orbit.  The  re- 
sult perplexed  him,  but  Bradley  had  a  mind 
open  to  suggestion,  and  capable  of  seeing,  in 
the  smallest  fact,  a  picture  of  the  largest.  He 
was  one  day  upon  ihe  Thames  in  a  boat,  and 
noticed  that,  as  long  as  his  course  remained 
unchanged,  the  vane  upon  his  mast-head 
showed  the  wind  to  be  blowing  constantly  in 
the  same  direction,  but  that  the  wind  ap- 
peared to  vary  with  every  change  in  the  di- 
rection of  his  boat.  "  Here,"  as  Whewell 
s.iy~,  "  was  the  image  of  his  case.  The  boat 
was  the  earth,  moving  in  its  orbit,  and  the 
wind  was  the  light  of  a  star." 

We  may  ask  in  passing,  what,  without  the 
faculty  which  formed  the  ''image,"  would 
Bradley's  wind  and  vane  have  been  to  him  ? 
'A  wind  and  vane,  and  nothing  more.  You 
will  immediately  understand  the  meaning  of 
Bradley's  discovery.  Imagine  yourself  in  a 
motionless  railway-train  with  a  shower  of  rain 
descending  vertically  downward.  The  mo- 
ment the  train  begins  to  move,  the  rain-drops 
begin  to  slant,  and  the  quicker  the  train  the 
greater  is  the  obliquity.  In  a  precisely  sim- 
ilar manner  the  rays  from  a  star  vertically 
overhead  are  caused  to  slant  by  the  motion  o'f 
the  earth  through  space.  Knowing  the 
speed  of  the  train,  and  the  obliquity  of  the 
falling  rain,  the  velocity  of  the  drops 
may  be  calculated  ;  and  knowing  the  speed 
of  the  earth  in  her  orbit,  and  the  obliquity  of 
the  rays  due  to  this  cause,  we  can  calculate 
just  as  easily  the  velocity  of  light.  Bradley 
did  this,  and  the  "aberration  of  light,"  as  his 
discovery  is  called,  enabled  him  to  assign  to 
it  a  velocity  almost  identical  with  that  de- 
duced by  Roemer  from  a  totally  different 
method  of  observation.  Subsequently  Fizeau, 
employing  not  planetary  or  stellar  distances, 
but  simply  the  breadth  of  the  city  of  Paris, 
determined  the  veloci  y  of  light  :  while  after 
him  Foucault — a  man  of  the  rarest  mechanical 
genius — solved  the  problem  without  quitting 
his  private  room. 

Up  to  his  demonstration  of  the  composition 
of  white  light,  Newton  had  been  everywhere 
triumphant — triumphant  in  the  heavens,  tri- 
umphant on  the  earth,  and  his  subsequent 
experimental  work  is  for  the  most  part  of 
immortal  value.  But  infallibility  is  not  the 
gift  of  man,  and,  soon  after  his  discovery  of 
the  nature  of  white  light,  Newton  proved 
himself  human.  He  supposed  that  refraction 
and  dispersion  went  hand  in  hand,  and  that 
you  could  not  abolish  the  one  without  at  the 
same  time  abolishing  the  other.  Here  Dol- 
land  corrected  him  But  Newton  committed 
a  graver  error  than  this.  Science,  as  I 
sought  to  make  clear  to  you  in  our  second 
lecture,  is  only  in  part  a  thing  of  the  senses. 
The  roots  of  phenomena  are  embedded  in  a  j 
region  beyond  the  reach  of  the  sen  es,  and  \ 
less  than  the  root  of  the  matter  will  never  ' 


satisfy  the  scientific  mind.  We  find,  accord- 
ingly, in  this  career  of  optics,  the  greatest 
minds  constantly  yearning  to  pass  from  the 
phenomena  tD  their  causes — to  explore  them 
to  their  hidden  roots.  They  thus  entered 
the  region  of  theory,  and  here  Newton, 
though  drawn  from  time  to  time  towards  the 
truth,  was  drawn  still  more  strongly  towards 
the  error,  and  made  it  his  substantial  choice. 
His  experiments  are  imperishable,  but  his 
theory  has  pa-sed  away.  For  a  century  it 
stood  like  a  dam  across  the  course  of  discov- 
ery ;  but,  like  all  barriers  that  rest  upon 
authority,  and  no.  upon  truth,  the  pressure 
from  behind  increased,  and  eventually  swept 
the  barrier  away.  This,  as  you  know,  was 
done  mainly  through  the  labors  of  Thomas 
Young,  and  his  illustrious  French  fellow- 
worker  Fresnel. 

In  1808,  Malus,  looking  through  Iceland 
spar  at  the  sun  reflected  from  the  window  of 
the  Luxembourg  Palace  in  Paris,  discovered 
the  polarization  of  light  by  reliection.  In 
1811  Arago  discovered  the  splendid  chro- 
matic phenomena  which  we  have  had  illus- 
trated by  plates  of  gypsum  i::  polarized  ligtit ; 
he  also  discovered  the  rotation  of  tne  plane 
of  polarization  by  quartz-crystals.  In  1813 
Seebeck  discovered  the  polarization  of  light 
by  tourmaline.  That  same  year  Brewscer 
discovered  those  magnificent  bane's  of  color 
that  surround  the  axes  of  biaxal  crystals.  In 
1814  Wollaston  discovered  the  rings  of  Ice- 
land spar.  All  these  effects,  whkh,  without 
a  theoretic  clue,  would  leave  the  human 
mind  in  a  hopeless  jungle  of  phenomena 
without  harmony  or  relation,  were  organically 
connected  by  the  theory  of  undulation.  The 
theory  was  applied  and  verified  in  all  direc- 
tions, Airy  being  especially  conspicuous  for 
the  severity  and  conclusiveness  of  his  proofs. 
The  most  remarkable  verification  fell  to  the 
lot  of  the  late  Sir  William  Hamilton,  of  Dub- 
lin, a  profound  mathematician,  who,  taking 
up  the  theory  where  Fresnel  had  left  it,  ar- 
rived at  the  conclusion  that,  at  foar  bpjjial 
points  at  the  surface  of  the  ether-wave  in 
double-refracting  crystals,  the  ray  was  divided 
not  into  two  parts,  but  into  an  infinite  num- 
ber of  parts  ;  forming  at  these  points  a  con- 
tinuous conical  envelope  in3tead  of  two 
images.  No  human  eye  had  ever  seen  this 
envelope  when  Sir  William  Hamilton  inferred 
its  existence.  Turning  to  nis  fri  nJ  Dr. 
Lloyd,  he  asked  him  to  test  experimentally 
the  truth  of  his  theoretic  conclusion.  Lloyd, 
taking  a  crystal  of  arragomtc,  and  following 
with  the  most  scrupulous  exactness  the  indi- 
cations of  theory,  cutting  the  crystal  wheie 
theory  said  it  ought  to  be  cut,  observing  it 
..here  theory  said  it  ought  to  be  observed, 
found  the  luminous  envelope  which  had  pre 
viousiy  been  a  mere  idea  in  the  mind  of  the; 
mathematician. 

Nevertheless  this  great  theory  cf  undula- 
tion, like  many  another  truth,  which  i  i  the 
long-rua  has  proved  a  blessi  ig  ti  humanity. 


SIX  LECTURES   ON  LIGHT. 


had  to  establish,  by  hot  conflict,  its  right  to 
existence.  Great  names  were  arrayed  against 
it.  Ic  had  been  enunciated  by  Hooke,  it  had 
been  applied  by  lluyghens.  it  had  been  de- 
fended by  Euler.  But  they  made  no  impres- 
sion. And,  indeed,  the  theory  in  their  hands 
was  more  an  analogy  than  a  demonstration. 
It  first  took  the  form  of  a  demonstrated  verity 
in  the  hand  of  Thomas  Young.  He  broug'ic 
the  waves  of  light  to  bear  upon  each  other, 
causing  them  to  support  each  other,  and  to 
extinguish  each  other  at  will.  From  their 
mutual  actions  he  determined  their  lengths, 
and  applied  his  determinations  in  all  direc- 
tions. He  showed  t  at  the  standing  difficulty 
of  polarization  might  be  embraced  by  the 
theory.  After  him  came  Fresnel,  whose 
transcendent  mathematical  abilities  enabled 
him  to  give  the  theory  a  generality  unattained 
by  Young.  He  grasped  the  theory  in  its 
entirety  ,  followed  the  ether  into  its  eddies 
and  estuaries  in  the  hearts  of  crystals  of  the 
most  complicated  structure,  and  into  bodies 
subjected  to  strain  and  pressure.  He  showed 
that  the  facts  discovered  by  Malus,  Arago, 
Brewster,  and  Biot,  were  so  many  ganglia, 
so  to  speak,  of  his  theoretic  organism,  deriv 
ing  from  it  sustenance  and  explanation. 
\Vith  a  mind  too  strong  for  the  body  with 
which  it  was  associated,  that  body  became  a 
wreck  long  before  it  had  become  old,  and 
Fresnel  died,  leaving,  however,  behind  him  a 
name  immortal  in  the  annals  of  science. 

One  word  more  I  should  like  to  say  regard- 
ing Fresnel.  There  are  things,  ladies  and 
gentlemen,  better  even  than  science.  There 
are  matters  of  the  character  as  well  as  matters 
of  Uu  intellect,  and  it  is  always  a  pleasure  to 
those  who  wish  to  think  well  01  human  na- 
ture, when  high  intellect  and  upright  charac- 
ter are  combined.  They  were,  I  believe, 
combined  in  this  young  Frenchman.  In 
thosi  hot  conflicts  of  the  undulatory  theory, 
he  stood  forth  as  a  man  of  integrity,  claiming 
no  more  than  his  right,  and  ready  to  concede 
their  rights  to  others.  He  at  once  recognized 
and  acknowledged  the  merits  of  Thomas 
Young.  Indeed,  it  was  he,  and  his  fellow- 
countryman  Arago,  who  first  startled  England 
into  the  consciousness  of  the  injustice  done 
to  Young  in  the  Edinburgh  Review.  1  should 
like  to  read  you  a  brief  extract  from  a  letter 
written  by  Fresnel  to  Young  in  1824,  as  it 
throws  a  pleasant  light  upon  the  character  of 
the  Frenc  )  philosopher.  '*  For  a  long  time," 
says  Fresnel,  "that  sensibility,  or  that 
vanity,  which  people  call  love  of  glory,  has 
been  much  blunted  in  me.  I  labor  much 
Jess  to  catch  the  suffrages  of  the  public  than 
to  obtain  that  inward  approval  which  has 
always  been  the  sweetest  reward  of  my  efforts. 
Without  doubt,  in  moments  of  disgust  and 
discouragement,  I  have  often  needed  the  spur 
of  vanity  to  excite  me  to  pursue  my  researches. 
But  all  the  compliments  I  have  received  from 
Arago,  De  la  Place,  and  Biot,  never  gave  me 
so  much  pleasure  as  the  discovery  of  a  theo- 


retic truth,  or  the  confirmation  of  a  calcu'a- 
tion  by  experiment." 

This,  ladies  and  gentlemen,  is  the  core  of 
the  whole  matter  as  regards  science.  It 
must  be  cultivated  for  its  own  sake,  for  the 
pure  love  of  truth,  rather  than  for  the  ap- 
plause or  profit  that  it  brings.  And  now  my 
I  occupation  in  America  is  wellnigh  g'7ie". 
Still  I  will  bespeak  your  tolerance  for  a  few 
concluding  remarks  in  reference  to  the  me;i 
who  have  bequeathed  to  us  the  vast  body  of 
knowledge  of  which  I  have  sought  to  give 
you  some  faint  idea  in  these  lectures.  What 
was  the  motive  that  spurred  them  on?  what 
the  prize  of  their  high  calling  for  whici  they 
struggled  so  assiduously?  What  urged  them 
to  those  battles  and  those  victories  over  reti- 
cent Nature  which  have  become  the  heritage 
of  the  human  race?  It  is  never  to  be  for- 
gotten that  not  one  of  those  great  investiga- 
tors, from  Aristotle  down  to  Stokes  and 
Kirchhoff,  had  any  practical  end  in  view, 
according  to  the  ordinary  definition  of  the 
word  "practical."  They  did  not  propose  to 
themselves  money  as  an  end,  and  knowledge 
as  a  means  of  obtaining  it.  For  the  most 
part,  they  nobly  reversed  this  process,  made 
knowledge  their  end,  and  such  money  as 
they  possessed  the  means  of  obtaining  it. 

We  may  see  to-day  the  issues  of  their 
work  in  a  thousand  practical  forms,  and  this 
may  be  thought  sufficient  to  justify  i'c,  if  not 
ennoble  their  efforts.  But  they  did  not  work 
for  such  issues  ;  their  reward  wa:,  of  a  totally 
different  kind.  We  love  clothes,  we  love 
luxuries,  we  love  fine  equipages,  we  love 
money,  and  any  man  who  can  point  to  these 
as  the  result  of  his  efforts  in  life  justifies 
these  efforts  before  all  the  world.  In  Ameri- 
ca and  England  more  especially  he  is  a 
"  practical  "  man.  But  I  would  appeal  con- 
fidently to  this  assembly  whether  such  things 
exhaust  the  demands  of  human  nature?  The 
very  presence  here  for  six  inclerr.ent  nights 
of  this  audience,  embodying  so  much  oi  the 
mental  force  and  refinement  of  this  great 
city,  is  an  answer  to  my  question  I  need 
not  tell  such  an  assembly  that  there  are  joys 
of  the  intellect  as  well  as  joys  of  the  body, 
or  that  these  pleasures  of  the  spirit  consti- 
tuted the  reward  of  our  great  investigators. 
Led  on  by  the  whisperings  of  natural  truth, 
through  pain  and  self-denial,  they  often  pur- 
sued their  work.  With  the  ruling  passion 
strong  in  death,  some  of  them,  when  no 
longer  able  to  hold  a  pen,  dictated  to  their 
friends  the  result  of  their  labors,  and  then 
rested  from  them  forever. 

Could  we  have  seen  these  men  at  work 
without  any  knowledge  of  the  consequences 
of  their  work,  what  should  we  have  thought 
of  them  ?  To  many  of  their  contemporaries 
it  would  have  appeared  simply  ridiculous  to 
see  men,  whose  name 3  are  now  stars  in  the 
firmanent  of  science,  straining  their  atten- 
tion to  observe  an  effect  of  experiment  al- 
most too  minute  for  detection.  To  the  un- 


SIX  LECTURES  ON  LI3I-IT. 


initiated,  they  might  well  appear  as  big 
children  playing  with  not  Very  amusing  toys. 
It  is  so  to  this  hour.  Could  you  watch  the 
true  investigator — your  Henry  or  your  Dra- 
per, ior  example — in  his  laboratory,  unless 
animated  by  his  spirit,  you  could  hardly  un- 
derstand what  keeps  him  there.  Many  of 
the  objects  which  rivet  his  attention  might 
appear  to  you  utterly  tiivial  ;  and,  if  you  were 
cO  step  forward  and  ask  him  what  is  the  use  of 
his  work,  the  chances  are  that  you  would 
confound  him.  He  might  not  be  able  to 
express  the  1:33  of  it  in  intelligible  terms. 
He  might  not  be  able  to  assure  you  that  it 
will  put  a  dollar  into  the  pocket  of  any 
human  being  living  or  to  come.  That 
scientific  discovery  may  put  not  only  dollars 
into  the  potkets  of  individuals,  but  millions 
into  the  exchequers  or  nations,  the  history  of 
science  amply  proves  ;  but  the  hope  of  its 
doing  so  never  was  and  never  can  be  the 
motive  power  of  the  investigator. 

I  know  that  1  run  some  risk  in  speaking 
thus  before  practical  men.  1  kno,v  what  De 
Tocqueville  says  of  you.  "  The  man  of  the 
North,"  he  says,  "has  not  only  experience, 
but  knowledge.  He,  howevers  does  not  care 
for  science  as  a  pleasure,  and  only  embraces 
i  with  avidity  when  it  leads  to  useful  appli- 
cations." But  what,  I  would  ask,  are  the 
hopes  of  useful  applications  which  have 
drawn  you  so  many  times  to  this  place  in 
ipite  of  snow-drifts  and  biting  cold?  What, 
1  may  ask,  is  the  origin  of  that  kindness 
which  drew  me  from  my  work  in  London  to 
address  you  here,  and  which,  if  I  permitted 
it,  would  send  me  home  a  millionaire  ?  Not 
because  I  had  taught  you  to  make  a  single 
cent  by  science,  am  I  among  you  to-night, 
but  because  I  tried  to  the  best  of  my  ability 
co  present  science  to  the  world  «s  an  intellec- 
tual good.  Surely  no  two  terms  were  ever 
so  distorted  and  misapplied  with  refe.ence  to 
man  in  his  higher  relations  as  these  terms 
ustful  and  practical.  As  if  there  were  no 
nakedness  of  the  mind  to  be  clothed  as  well 
as  naktdness  of  the  body — no  hunger  and 
thirst  of  the  intellect  to  satisfy.  Let  us  ex- 
pand the  definitions  of  these  terms  until  thty 
embrace  all  the  needs  of  man,  his  highest  in- ! 
teliectual  needs  inclusive.  It  is  specially  on  ' 
this  ground  of  its  administering  to  the  higher  I 
needs  of  intellect,  it  is  mainly  because  I  be- 
lieve it  to  be  wholesome  as  a  source  of  know- 
ledge, and  as  a  means  of  discipline,  that  1 
virge  the  claims  of  science  this  evening  upon 
your  attention. 

But.  with  reference  to  material  needs  and 
joys,  surely  pure  science  has  also  a  word  to 
say.  People  sometimes  s-pc-ak  as  if  steam  had 
not  been  studied  before  James  Watt,  or  elec- 
tricity before  Wheatstone  and  Morse  ;  where- 
as, in  point  of  fact,  Watt  and  Wheatstone 
and  Morse,  with  all  their  practicality,  were 
the  mere  outcome  of  antecedent  forces,  which 
acted  without  reference  to  practical  ends. 
This  also,  I  think,  me  its  a  moment's 


J  tion.  You  arc  delighted,  and  with  good 
(reason,  wilh  your  electric  telegrrphs,  prcud 
of  your  steam-engines  and  your  factories, 
and  charmed  with  the  productions  of  pho- 
tography. You  see  daily,  with  just  elation, 
the  creation  of  new  forms  of  industry — • 
new  powers  of  adding  to  the  wealth  and 
comfort  of  society.  Industrial  England  is 
heaving  with  forces  tending  to  this  end,  and 
the  pulse  of  industry  beats  still  stronger  in 
the  United  States.  And  yet,  when  analyzed, 
what  are  industrial  America  and  industrial 
England?  If  you  can  tolerate  freedom  of 
speech  on  my  part,  I  will  answer  this  ques- 
tion by  an  illustration.  Strip  a  strong  arm, 
and  regard  the  knotted  muscles  when  the 
hand  is  clenched  and  the  arm  bent.  Is  this 
exhibition  of  energy  the  work  of  the  muscle 
alone  ?  By  no  means.  The  muscle  is  the 
channel  of  an  influence,  without  which  it 
would  be  as  powerless  as  a  lump  of  plastic 
dough.  It  is  the  delicate  unseen  nerve  that 
unlocks  the  power  of  the  muscle.  And, 
without  those  filaments  of  genius  which  have 
been  shot  like  nerves  through  the  body  of 
society  by  the  original  discoverer,  industrial 
America  and  industrial  England  would,  I 
fear,  be  very  much  in  the  condition  of  that 
plastic  dough. 

At  the, present  time  there  is  a  cry  in  Eng- 
land for  technical  education,  and  it  is  the  ex- 
pression of  a  true  national  want  ;  but  there 
is  no  cry  for  original  investigation.  Still 
without  this,  as  surely  as  the  stream  dwindles 
when  the  spring  dries,  so  surely  will  "tech- 
nical education"  lose  all  force  of  growth,  all 
power  of  reproduction.  Our  great  investi- 
gators have  given  us  sufficient  work  for  a 
time  ,  but,  if  their  spirit  die  out,  we  shall 
find  ourselves  eventually  in  the  condition  of 
those  Chinese  mentioned  by  Ue  Tocqueville. 
who,  having  forgotten  the  scientific  origin  of 
what  they  did,  were  at  length  compelled  to 
copy  without  variation  the  inventions  of  an 
ancestry  who,  wiser  than  themselves,  had 
drawn  their  inspiration  direct  from  Nature. 

To  keep  society  as  regards  science  in 
healthy  play,  three  classes  of  workers  are 
necessary  :  Firstly,  the  investigator  of  natural 
truth,  whose  vocation  it  is  to  pursue  that 
truth,  and  extend  the  field  of  discovery  for 
the  truth's  own  sake,  and  without  reference 
to  practical  ends.  Secondly,  the  teacher  of 
natural  truth,  whose  vocation  it  is  to  give 
public  diffusion  to  the  knowledge  already  won 
by  the  discoverer.  Thirdly,  the  applier  of 
natural  truth,  whose  vocation  it  is  to  make 
scientific  knowledge  available  for  the  needs, 
comforts,  and  luxuries  of  life.  These  three 
classes  ought  to  coexist  and  interact.  Now, 
the  popular  notion  of  science,  both  in  this 
country  and  in  England,  often  relates,  not  to 
science  strictly  so  called,  but  to  ths  applica- 
tions of  science.  Such  applications,  espe- 
cially on  this  continent,  are  so  astounding — 
they  spread  themselves  so  largely  2nd  um- 
bragcously  before  the  public  eye — as  to  shut 


SIX  LECTURES  ON  LIGHT. 


out  from  view  those  workers  who  are  engaged 
in  the  quieter  ami  profounder  business  of 
original  investigation. " 

Take  the  electric  telegraph  as  an  example, 
which  has  been  repeatedly  forced  upon  my 
attention  of  late.  I  am  not  here  to  attenuate 
in  the  slightest  degree  the  services  of  those 
who,  in  England  and  America,  have  given 
the  telegraph  a  form  so  wonderfully  fitted  for 
public  use.  They  earned  a  great  reward, and 
assuredly  they  have  received  it.  But  I  should 
be  untrue  to  you  and  to  myself  if  I  failed  to 
tell  you  that,  however  high  in  particular  re- 
spects their  claims  and  qualities  may  be,  prac- 
tical men  did  not  discover  the  electric  tele- 
gn».ph.  The  discovery  of  the  electric  telegraph 
implies  the  discovery  of  electricity  itself,  and 
the  development  cf  its  laws  and  phenomena. 
Such  discoveries  are  not  made  by  practical 
men,  and  they  never  will  be  made  by  them, 
because  their  minds  are  beset  with  ideas 
which,  though  of  the  highest  value  from  one 
point  of  view,  are  not  those  which  stimulate 
the  original  discoverer. 

The  ancients  discovered  the  electricity  of 
amber ;  and  Gilbert,  in  the  year  1600,  ex- 
tended the  force  to  other  bodies.  Then  fol- 
lowed other  inquirers,  your  own  Franklin 
among  the  number.  But  this  form  of  elec- 
tricity, though  tried,  did  not  come  into  use 
for  telegraphic  purposes.  Then  appeared  the 
great  Italian,  Volto,  who  discovered  the 
source  of  electricity,  which  bears  his  name, 
and  applied  the  most  profound  iasight  and 
the  most  delicate  experimental  skill  to  its  de- 
velopment. Then  arose  the  man  who  added 
to  the  powers  of  his  intellect  all  the  graces  of 
the  human  heart,  Michael  Faraday,  the  dis- 
coverer of  the  great  domain  of  magneto- 
electricity.  CErsted  discovered  the  deflection 
of  the  magnetic  needle,  and  Arago  and  Stur- 
geo:i  the  magnetization  of  iron  by  the  elec- 
tric current.  The  voltaic  circuit  finally  found 
its  theoretic  Newton  in  Ohm,  while  Henry, 
of  Princeton,  who  had  the  sagacity  to  recog- 
nize the  merits  of  Ohm  while  they  were  still 
decried  in  his  own  country,  was  at  this  time 
in  the  van  of  experimental  inquiry. 

In  the  works  of  these  men  you  have  all 
the  materials  employed  at  this  hour  in  all  the 
forms  of  the  electric  telegraph.  Nay,  more  ; 
Gauss,  the  celebrated  astronomer,  and  Weber, 
the  celebrated  natural  philosopher,  both  pro- 
fessors in  the  University  of  Gottingen,  wish- 
ing to  establish  a  rapid  mode  of  communication 
between  the  observatory  and  the  physical 
cabinet  of  the  university,  did  this  by  means 
of  an  electric  telegraph.  The  force,  in 
short,  had  been  discovered,  it-  laws  investi- 
gated and  made  sure,  the  most  complete 
mastery  of  i's  phenomena  had  been  attained, 
nay,  its  applicability  to  teleg.aphic  purposes 
demonstrated,  by  men  whose  sole  reward  for 
their  labors  was  the  noble  joy  of  discovery, 
and  before  your  practical  men  appeared  at 
all  upon  the  scene. 

Are  we  to   ignore   all   this  ?     We  do  so  at 


our  peril.  For  I  say  agaia,  that,  behind  all, 
your  practical  applications,  there  is  a  region 
of  intellectual  action  to  which  practical  men 
have  rarely  contributed,  but  from  which  they 
draw  all  their  supplies.  Cut  them  off  from 
this  region,  and  they  become  eventually  help- 
less. In  no  case  is  the  adage  truer,  '"Other 
men  labored,  but  ye  are  entered  in  o  their 
labors,"  than  in  the  case  of  the  discoverer 
and  the  applier  of  natural  truth.  But  now  a 
word  on  the  other  side.  While  I  say  that 
practical  men  are  not  the  men  to  make  the 
necessary  antecedent  discoveries,  the  cases 
are  rare  in  which  the  discoverer  knows  how 
to  turn  his  labors  to  practical  account.  Dif- 
ferent qualities  of  mind  and  different  habits 
of  thought  are  needed  in  ;he  two  cases;  and, 
while  I  wish  to  give  emphatic  utterance  to 
the  claims  of  those  whose  claims,  owing  to 
the  simple  fact  of  their  intellectual -elevation, 
arc  often  misunderstood,  I  am  not  here  to 
exalt  the  one  class  of  workers  at  the  expense 
of  the  other.  They  are  the  necessary  supple- 
ments of  each  other;  but  remember  that  one 
class  is  sure  to  be  taken  care  of.  All  the  ma- 
terial rewards  of  society  are  already  within 
their  reac.i;  but  it  is  at  our  peril  that  we  neg- 
lect to  provide  opportunity  for  those  studies 
and  pursuits  which  have  no  such  rewards, 
and  from  which,  therefore,  the  rising  genius 
of  the  country  is  incessantly  tempted  away. 

Pasteur,  one  of  the  most  eminent  members 
of  the  Institute  of  Fiance,  in  accounting  for 
the  disastrous  overthrow  of  his  country  and 
the  predominance  of  Germany  in  the  late 
war,  expresses  himself  thus:  "  Few  persons 
comprehend  the  real  origin  of  the  marvels  of 
industry  and  the  wealth  of  nations.  I  need 
no  further  proof  of  this  than  the  employment, 
more  and  more  frequent  in  official  language, 
and  in  writing  of  all  sorts,  of  the  erroneous 
expression  applied  science.  The  abandon- 
ment of  scientific  c  reers  by  men  capable  oi" 
pursuing  them  with  distinction  was  recently 
complained  of  in  the  presence  of  a  minister 
of  the  greatest  talent.  This  statesman  en- 
deavored to  show  that  we  ought  not  to  be 
surprised  at  this  result,  because  /;/  our  day 
the  reign  of  theoretic  science  yielded  place  to 
tJiat  of  applied  science.  Nothing  coald  bs 
more  erroneous  than  this  opinion,  nothing,  I 
venture  to  say,  more  dangerous,  even  to 
practical  life,  than  the  consequences  whicn 
might  How  from  these  words.  They  have 
rested  on  my  mind  as  a  proof  of  the  imperi- 
ous necessity  of  reform  in  our  superior  edu- 
cation. There  exists  no  category  of  the 
sciences  to  which  the  name  of  applied  science 
could  be  given.  ll'e  have  science,  and  the 
applications  of  science  which  are  united  to- 
gether as  the  tree  and  its  .\ruit." 

And  Cuvier,  the  great  comparative  anato- 
mist, writes  thus  upon  the  same  theme- 
"These  grand  practical  innovations  are  the 
mere  applications  of  truths  of  a  higher  order, 
not  sought  with  a  practical  intent,  but  which 
were  pursued  for  their  own  sake,  and  solely 


SIX  LECTURES  ON  LIGHT. 


through  an  ardor  for  knowledge.  Those 
who  applied  them  could  not  have  discovered 
them;  those  who  discovered  them  had  no  in- 
clination to  pursue  them  to  a  practical  end. 
Engaged  in  the  high  regions  whither  their 
thoughts  had  carried  them,  they  hardly  per- 
ceived these  practical  issues,  though  born  of 
their  own  deeds.  These  rising  workshops, 
these  peopled  colonies,  those  ships  which  fur- 
row the  seas — this  abundance,  this  luxury, 
this  tumult — all  th's  comes  from  discoverers 
in  science.,  and  it  all  remains  strange  to 
them.  At  the  point  where  science  merges 
into  practice,  they  abandon  it;  it  concerns 
them  no  more." 

When  the  Pilgrim  Fathers  landed  at  Ply- 
mouth Rock,  and  when  Penn  made  his  treaty 
with  the  Indians,  the  new-comers  had  to 
bui'.d  their  houses,  to  chasten  the  earth  into 
cultivation,  and  to  take  care  of  their  souls. 
In  such  a  community,  science,  in  its  more 
abstract  forms,  was  not  to  be  thought  cf. 
And,  at  the  present  hour,  when  your  hardy 
Western  pioneers  stand  face  to  face  with 
stubborn  Nature,  piercing  the  mountains 
and  subduing  the  forest  and  the  prairie,  the 
pursuit  of  science,  for  its  own  sake,  is  not  to 
be  expected.  The  first  need  of  man  is  food 
nnd  shelter  ;  but  a  vast  portion  of  this  con- 
tinent is  already  raised  far  beyond  this  need. 
The  gentlemen  of  New  York,  Brooklyn,  Bos- 
ton, Philadelphia,  Baltimore,  and  Washing- 
ton, have  already  built  their  houses,  and  very 
beautiful  they  are  ;  they  have  also  secured 
their  dinners,  to  the  excellence  of  which  I 
can  also  bear  testimony.  They  have,  in 
fact,  reached  that  precise  condition  of  well- 
being  and  independence  when  a  culture,  as 
}  i^'h  as  humanity  has  yet  reached,  may  be 
justly  demanded  at  their  hands.  They  have 
reached  that  maturity,  as  possessors  of  wealch 
and  leisure,  when  the  investigator  of  natural 
truth,  for  the  truth's  own  sake,  ought  to  find 
among  them  promoters  and  protectors. 

Among  the  many  grave  problems  before 
them  they  have  this  to  solve,  whether  a  re- 
public is  able  to  foster  the  highest  forms  of 
genius.  You  are  familiar  with  the  writings 
of  De  Tocqueville,  and  must  be  aware  of  the 
intense  sympathy  which  he  felt  for  your  insti- 
tutions ;  and  this  sympathy  is  all  the  more 
valuable,  from  the  philosophic  candor  with 
which  he  points  out,  not  only  your  merits, 
but  your  defects  and  dangers.  Now,  if  I 
come  here  to  speak  of  science  in  America  in 
a  critical  and  captious  spirit,  an  invisible 
radiation  from  my  words  and  manner  will 
enable  you  to  find  me  out,  and  will  guide 
your  treatment  of  me  to-night.  But,  if  I,  in 
no  unfriendly  spirit — in  a  spirit,  indeed,  th~ 
reverse  of  unfriendly — venture  to  repeat  be- 
fore you  what  this  great  historian  and  analyst 
of  democratic  institutions  said  of  America,  I 
am  persuaded  that  you  will  hear  me  out.  He 
wrote  some  three-and-twenty  years  ago,  and 
perhaps  would  not  write  the  same  to-day  ; 
but  it  will  do  nobody  any  harm  to  have  his  I 


words  repeated,  and,  if  necessary,  laid  to 
heart.  In  a  work  published  in  1 850,  he  says : 
"  It  must  be  confessed  that,  among  the  civil- 
ized peoples  of  our  ^ge,  there  are  few  in 
which  the  highest  sciences  have  made  so 
little  progress  as  in  the  United  States."* 
He  declares  his  conviction  that,  had  you  been 
alone  in  the  universe,  you  would  speedily 
have  discovered  that  you  cannot  long  make 
progress  in  practical  science,  without  culti- 
vating theoretic  science  at  the  same  time. 
But,  according  to  De  Tocqueville,  you  are 
'  not  thus  alone.  He  refuses  to  separate 
America  from  its  ancestral  home  ;  and  it  is 
here,  he  contends,  that  you  collect  the  treas- 
ures of  the  intellect,  without  taking  the 
trouble  to  create  them. 

De  Tocqueville  evidently  doubts  the  ca- 
pacity of  a  democracy  to  1  osier  genius  as  it 
was  fostered  in  the  ancient  aristocracies. 
"  The  future,"  he  says,  "will  prove  whether 
the  passion  for  profound  knowledge,  so  rare 
and  so  fruitful,  can  be  born  and  developed 
so  readily  in  democractic  societies  as  in  aris- 
tocracies. As  for  me,"  he  continues,  "  I 
can  hardly  believe  it."  He  speaks  of  the  un- 
quiet feverishness  of  democratic  commu- 
nities, not  in  times  of  great  excitement,  for 
such  times  may  give  an  extraordinary  impe- 
tus to  ideas,  but  in  times  of  peace.  There 
is  then,  he  says,  "a  small  and  uncomfortable 
agaitation,  a  sort  of  incessant  attrition  of 
man  against  man,  which  troubles  and  dis- 
tracts the  mind  without  imparting  to  it  cither 
animation  or  elevation."  It  rests  with  you 
to  prove  whether  these  things  are  necessarily 
so — whether  the  highest  scientific  genius  can- 
not find  in  the  midst  of  you  a  tranquil  homo. 
I  should  be  loath  to  gainsay  so  keen  an  ob- 
server and  so  profound  a  political  writer,  but, 
since  my  arrival  in  this  country,  I  have  been 
unable  to  see  anything  in  the  constitution  of 
society  to  prevent  a  Sstudent  with  the  rocl;  ot 
the  matter  in  him  from  bestowing  the  most 
steadfast  devotion  on  pure  science.  If  great 
scientific  results  are  not  achieved  in  Ameri- 
ca, it  is  not  to  the  small  agitations  of  society 
that  I  should  be  disposed  to  ascribe  the  de- 
fect, but  to  the  fact  that  the  men  among  you 
who  possess  the  endo.vments  necessary  for 
scientific  inquiry  are  laden  with  duties  of  ad- 
ministration or  tuition  so  heavy  as  to  be 
utterly  incompatible  with  the  continuous  and 
tranquil  meditation  which  original  investi- 
gation demands.  It  may  well  be  aske 
whether  Henry  would  have  been  transformer 
into  an  administrator,  or  whether  Draper 
would  have  forsaken  science  to  write  history, 
if  the  original  investigator  had  been  honored 
as  he  ought  to  be  in  this  land  ?  I  hardly 
think  they  would.  Still  I  do  not  think  thia 


*  II  faut  reconnaitre,  que  parmis  les  peuples  civil- 
ises de  nos  jours,  il  en  esc  p'eu  chez  qui  les  hautes 
sciences  aient  fait  moins  de  progres  qu'aux  Etats- 
Unis,  ou  qui  aient  fourni  moins  de  grands  artisres, 
de  poetesillustres,  et  de  celebrcs  ecrivains.  (Ds  U 
Democratic  en  Amerique,  etc.,  toius  ii.,  p.  36.) 


SIX  LECTURES  ON  LIGHT. 


state  of  things  likely  to  last.  In  America 
there  is  a  willingness  ou  the  part  of  individu- 
als to  devote  their  fortunes,  in  the  matter  of 
education,  to  the  service  of  the  common- 
wealth, which  is  without  a  parallel  elsewhere; 
and  this  willingness  requires  but  wise  di- 
rection to  enable  you  effectually  to  wipe  away 
the  reproach  of  De  Tocqu^ville. 

Your  most  difficult  problem  will  be  no1"  to 
build  institutions,  but  to  make  men  ;  not  to 
form  the  body,  but  to  find  tne  spiritual  em- 
bers which,  shall  kindle  within  that  body 
a  living  soul.  You  have  scientific  genius 
among  you ;  not  sown  broadcast,  believe 
me,  but  still  '  cattered  here  and  there.  Take 
a'l  r.nnecessary  impediments  out  of  its  way. 
Drawn  by  your  kindness  I  have  come  here  to 
give  these  lectures,  and,  now  that  my  visit  to 
America  has  become  almost  a  thing  of  the 
past,  I  look  back  upon  it  ns  a  memory  with- 
out a  stain.  No  lecturer  was  ever  rewarded 


as  I  have  been.  From  this  vantage-ground, 
howtver,  let  me  remind  you  that  the  work  of 
the  lecturer  is  not  the  highest  work  ;  that  in 
science,  the  lecturer  is  usually  the  distributor 
of  intellectual  wealth  amassed  by  better  men. 
It  is  not  solely,  or  even  chiefly,  as  lecturers, 
but  as  investigators,  that  your  men  of  genius 
ought  to  be  employed.  Keep  your  sympa- 
thetic eye  upon  the  originator  of  knowledge. 
Give  him  the  freedom  necessary  for  his  re- 
searches, not  overloading  him  either  with  the 
duties  of  tu  tion  or  of  administration,  not  de- 
manding from  him  so-called  practical  results 
— above  all  things,  avoiding  that  question 
which  ignorance  so  often  addresses  to  genius, 
"  What  is  the  use  of  your  work  ?  "  Let  him 
make  truth  his  object,  however  unpractical 
for  the  time  being  that  truth  may  appear. 
If  you  cast  your  bread  thus  upon  the  waters, 
then  be  assured  it  will  return  to  you,  though 
it  may  be  after  many  days. 


CONTENTS. 


Xscr.     I.— INTRODUCTORY 2 

II.— Origin  of  Physical  Theories    ...  8 

III — Relation  of  Theories  to  Experience  .  18 
IV. — Chromatic  Phenomena  produced  by 

Crystals 27 


LECT.     V. — Range  of  Vision  and  Range  of  Ra- 
diation   34 

VI. — Spectrum  Analysis.    ......    45 


Tyndall, 

J. 

'  Select 
Tyndall 

works  of  Jo] 

in 

~~s* 

M300991 

QtV 

Y 


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